better help message for MissingPositional error (#10949)

Added "Use `--help` for more information." to the help of
MissingPositional error
- this PR should close
[#10946](https://github.com/nushell/nushell/issues/10946)

**Before:**

![image](https://github.com/nushell/nushell/assets/1835944/629aeaae-e985-41aa-a791-05ef062e988e)

**After:**

![image](https://github.com/nushell/nushell/assets/1835944/0bc1868c-ffed-4440-ad98-2cf29aa8c656)



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---------

Co-authored-by: Denis Zorya <denis.zorya@trafigura.com>

crates/nu-protocol/src/parse_error.rs

@@ -339,7 +339,10 @@ pub enum ParseError {
     OnlyLastFlagInBatchCanTakeArg(#[label = "only the last flag can take args"] Span),
 
     #[error("Missing required positional argument.")]
-    #[diagnostic(code(nu::parser::missing_positional), help("Usage: {2}"))]
+    #[diagnostic(
+        code(nu::parser::missing_positional),
+        help("Usage: {2}. Use `--help` for more information.")
+    )]
     MissingPositional(String, #[label("missing {0}")] Span, String),
 
     #[error("Missing argument to `{1}`.")]