Port command examples to long option (#10596)

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# Description

Long options are preferable over short ones for documentation.
This PR ports some command examples to exclusively use long options.
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# User-Facing Changes
 
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crates/nu-command/src/filesystem/cp.rs

@@ -359,12 +359,12 @@ impl Command for Cp {
             },
             Example {
                 description: "Recursively copy dir_a to dir_b",
-                example: "cp -r dir_a dir_b",
+                example: "cp --recursive dir_a dir_b",
                 result: None,
             },
             Example {
                 description: "Recursively copy dir_a to dir_b, and print the feedbacks",
-                example: "cp -r -v dir_a dir_b",
+                example: "cp --recursive --verbose dir_a dir_b",
                 result: None,
             },
             Example {
@@ -374,7 +374,7 @@ impl Command for Cp {
             },
             Example {
                 description: "Copy only if source file is newer than target file",
-                example: "cp -u a b",
+                example: "cp --update a b",
                 result: None,
             },
         ]

crates/nu-command/src/filesystem/ucp.rs

@@ -68,12 +68,12 @@ impl Command for UCp {
             },
             Example {
                 description: "Recursively copy dir_a to dir_b",
-                example: "ucp -r dir_a dir_b",
+                example: "ucp --recursive dir_a dir_b",
                 result: None,
             },
             Example {
                 description: "Recursively copy dir_a to dir_b, and print the feedbacks",
-                example: "ucp -r -v dir_a dir_b",
+                example: "ucp --recursive --verbose dir_a dir_b",
                 result: None,
             },
             Example {

crates/nu-command/src/viewers/table.rs

@@ -134,7 +134,7 @@ impl Command for Table {
         vec![
             Example {
                 description: "List the files in current directory, with indexes starting from 1.",
-                example: r#"ls | table -n 1"#,
+                example: r#"ls | table --start-number 1"#,
                 result: None,
             },
             Example {

crates/nu-explore/src/explore.rs

@@ -113,13 +113,13 @@ impl Command for Explore {
             },
             Example {
                 description: "Explore a list of Markdown files' contents, with row indexes",
-                example: r#"glob *.md | each {|| open } | explore -i"#,
+                example: r#"glob *.md | each {|| open } | explore --index"#,
                 result: None,
             },
             Example {
                 description:
                     "Explore a JSON file, then save the last visited sub-structure to a file",
-                example: r#"open file.json | explore -p | to json | save part.json"#,
+                example: r#"open file.json | explore --peek | to json | save part.json"#,
                 result: None,
             },
         ]