Improve json-from-wast -h output (bytecodealliance#1893)

* Improve `json-from-wast -h` output

Don't render a long variable name placeholder as
`ALLOW_CONFUSING_UNICODE` and instead render it just as `ALLOW`.

* Update src/bin/wasm-tools/json_from_wast.rs

Co-authored-by: Nick Fitzgerald <fitzgen@gmail.com>

---------

Co-authored-by: Nick Fitzgerald <fitzgen@gmail.com>

Author: alexcrichton

src/bin/wasm-tools/json_from_wast.rs

@@ -54,7 +54,7 @@ pub struct Opts {
     ///
     /// This is defaulted to `true` to enable parsing all upstream spec tests
     /// but can be disabled if desired too.
-    #[clap(long)]
+    #[clap(long, value_name = "true|false")]
     allow_confusing_unicode: Option<bool>,
 }