Editorial: use more idiomatic "if ..., then ..." phrasing

Author: foolip

fullscreen.bs

@@ -273,7 +273,7 @@ when invoked, must run these steps:
  <li><p>Return <var>promise</var>, and run the remaining steps <a>in parallel</a>.
 
  <li>
-  <p>If <var>error</var> is false: Resize <var>pendingDoc</var>'s
+  <p>If <var>error</var> is false, then resize <var>pendingDoc</var>'s
   <a>top-level browsing context</a>'s <a>active document</a>'s viewport's dimensions, optionally
   taking into account <var>options</var>'s <code>navigationUI</code> member:
   <!-- cross-process -->