doc: fix grammatical mistake

PR-URL: #52808
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Daeyeon Jeong <[email protected]>

Author: codershiba

doc/api/webstreams.md

@@ -1029,7 +1029,7 @@ changes:
     description: This class is now exposed on the global object.
 -->
 
-The `WritableStreamDefaultController` manage's the {WritableStream}'s
+The `WritableStreamDefaultController` manages the {WritableStream}'s
 internal state.
 
 #### `writableStreamDefaultController.error([error])`