Fix JSDoc grammar typo: 'returns a undefined' → 'returns undefined' (#63525)

Co-authored-by: Tech-Savvy Builder <tech@crypto-nite.com>

src/lib/es2015.symbol.d.ts

@@ -19,7 +19,7 @@ interface SymbolConstructor {
 
     /**
      * Returns a key from the global symbol registry matching the given Symbol if found.
-     * Otherwise, returns a undefined.
+     * Otherwise, returns undefined.
      * @param sym Symbol to find the key for.
      */
     keyFor(sym: symbol): string | undefined;