Fix ReadableStream cancel error message (#37282)

Align error message with WHATWG Streams spec by changing:  
- `'stream is not locked'` to `'stream is locked'` 

This will match the step 2 of the readablestream cancel method that
requires the correct TypeError when the stream is locked.

Signed-off-by: Alice Okingo <annekadiso@gmail.com>

components/script/dom/readablestream.rs

@@ -1926,7 +1926,7 @@ impl ReadableStreamMethods<crate::DomTypeHolder> for ReadableStream {
             // If ! IsReadableStreamLocked(this) is true,
             // return a promise rejected with a TypeError exception.
             let promise = Promise::new(&global, can_gc);
-            promise.reject_error(Error::Type("stream is not locked".to_owned()), can_gc);
+            promise.reject_error(Error::Type("stream is locked".to_owned()), can_gc);
             promise
         } else {
             // Return ! ReadableStreamCancel(this, reason).