Remove unnecessary echo uses from examples (#7500)

`echo` tends to confuse new Nu users; they expect it to work like
`print` when it just passes a value to the next stage of the pipeline.

We haven't quite figured out what to do about `echo` in the long run,
but I think a good start is to remove `echo` from command examples where
it would be unnecessary and arguably unidiomatic.

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

@@ -194,7 +194,7 @@ impl Command for Open {
             },
             Example {
                 description: "Open a file, using the input to get filename",
-                example: "echo 'myfile.txt' | open",
+                example: "'myfile.txt' | open",
                 result: None,
             },
             Example {