Update index.html

Co-authored-by: Reilly Grant <[email protected]>

index.html

@@ -1513,7 +1513,7 @@ <h2>
             [=Window/browsing context=] to:
               <ol>
                 <li>If |coordinates| is not given, clear the system location
-                and thus making the position unavailable.
+                and thus make the position unavailable.
                 </li>
                 <li>Otherwise, set the system coordinates to match the values
                 of |coordinates|.