[css-ui-4] Remove redundant 's', three times (#4243)

"to incorporates, extends, and supersedes these."
becomes
"to incorporate, extend, and supersede these."

resolves #3976

Author: ewilligers

css-ui-4/Overview.bs

@@ -104,7 +104,7 @@ and <a href="https://www.w3.org/TR/CSS2/ui.html">Chapter 18 of CSS2</a> [[CSS21]
 introduced several user interface related properties and values.
 <a href="https://www.w3.org/TR/2000/WD-css3-userint-20000216">User Interface for CSS3 (16 February 2000)</a> introduced several new user interface related features.
 
-[[CSS-UI-3]] was later introduced to incorporates, extends, and supersedes these.
+[[CSS-UI-3]] was later introduced to incorporate, extend, and supersede these.
 This specification continues this work, and in turn replaces [[CSS-UI-3]].