[css-timing-1] Rename <single-timing-function> to <timing-function>

See #2028.

Author: birtles

css-timing-1/Overview.bs

@@ -89,7 +89,7 @@ follow.
 
 The syntax for specifying a [=timing function=] is as follows:
 
-<div class="prod"><dfn type>&lt;single-timing-function&gt;</dfn> =
+<div class="prod"><dfn type>&lt;timing-function&gt;</dfn> =
   ''linear'' |
   <<cubic-bezier-timing-function>> |
   <<step-timing-function>> |
@@ -104,7 +104,7 @@ meaning that its [=output progress value=] is equal to the
 [=input progress value=] for all inputs.
 
 The syntax for the [=linear timing function=] is simply the
-<dfn dfn-type=value for=single-timing-function>linear</dfn> keyword.
+<dfn dfn-type=value for=timing-function>linear</dfn> keyword.
 
 
 Cubic B&eacute;zier timing functions: ''ease'', ''ease-in'', ''ease-out'', ''ease-in-out'', ''cubic-bezier()'' {#cubic-bezier-timing-functions}