Fix examples about RFC3339 format in date now and format date. (#15563)

Replace example on `date now | debug` with `date now | format date
"%+"`. Add RFC3339 "%+" format string example on `format date`.

Users can now find how to format date-time to RFC3339.

FIXES: #15168

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# Description
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Documentation will now provide users examples on how to print RFC3339
strings.

# User-Facing Changes
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helps us keep track of breaking changes. -->

Corrects documentation.

# Tests + Formatting
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crates/nu-command/src/date/now.rs

@@ -38,10 +38,16 @@ impl Command for DateNow {
     fn examples(&self) -> Vec<Example> {
         vec![
             Example {
-                description: "Get the current date and display it in a given format string.",
+                description: "Get the current date and format it in a given format string.",
                 example: r#"date now | format date "%Y-%m-%d %H:%M:%S""#,
                 result: None,
             },
+            Example {
+                description:
+                    "Get the current date and format it according to the RFC 3339 standard.",
+                example: r#"date now | format date "%+""#,
+                result: None,
+            },
             Example {
                 description: "Get the time duration since 2019-04-30.",
                 example: r#"(date now) - 2019-05-01"#,
@@ -53,7 +59,8 @@ impl Command for DateNow {
                 result: None,
             },
             Example {
-                description: "Get current time in full RFC 3339 format with time zone.",
+                description:
+                    "Get current time and format it in the debug format (RFC 2822 with timezone)",
                 example: r#"date now | debug"#,
                 result: None,
             },

crates/nu-command/src/strings/format/date.rs

@@ -61,6 +61,14 @@ impl Command for FormatDate {
                     Span::test_data(),
                 )),
             },
+            Example {
+                description: "Format a given date-time according to the RFC 3339 standard.",
+                example: r#"'2021-10-22 20:00:12 +01:00' | into datetime | format date "%+""#,
+                result: Some(Value::string(
+                    "2021-10-22T20:00:12+01:00".to_string(),
+                    Span::test_data(),
+                )),
+            },
             Example {
                 description: "Format the current date-time using a given format string.",
                 example: r#"date now | format date "%Y-%m-%d %H:%M:%S""#,