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/system/nu_check.rs

@@ -193,7 +193,7 @@ impl Command for NuCheck {
             },
             Example {
                 description: "Parse a string as script",
-                example: "echo $'two(char nl)lines' | nu-check ",
+                example: "$'two(char nl)lines' | nu-check ",
                 result: None,
             },
             Example {