Minor errata for 1.4.13 Content on Hover or Focus (#4469)

Closes: #4303

Description: updated 1.4.13 Content on Hover or Focus, to clarify that
all the requirements must be met.

Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>
Co-authored-by: Mike Gower <mikegower@gmail.com>
(cherry picked from commit c3a8202)

Author: 3 people

guidelines/sc/21/content-on-hover-or-focus.html

@@ -4,7 +4,7 @@ <h4>Content on Hover or Focus</h4>
 
    <p class="conformance-level">AA</p>
 
-    	<p>Where receiving and then removing pointer hover or keyboard focus triggers additional content to become visible and then hidden, the following are true:</p>
+    	<p>Where receiving and then removing pointer hover or keyboard focus triggers additional content to become visible and then hidden, all of the following are true:</p>
 
   <dl>