[css-timing-1] Move <single-timing-function> production

This addresses the second issue raisd in w3c#2027.

Author: birtles

css-timing-1/Overview.bs

@@ -87,6 +87,14 @@ input which is defined subsequently.
 This specification defines four types of timing functions whose definitions
 follow.
 
+The syntax for specifying a [=timing function=] is as follows:
+
+<div class="prod"><dfn type>&lt;single-timing-function&gt;</dfn> =
+  ''linear'' |
+  <<cubic-bezier-timing-function>> |
+  <<step-timing-function>> |
+  <<frames-timing-function>></div>
+
 
 The linear timing function: ''linear'' {#linear-timing-function-section}
 ------------------------------------------------------------------------
@@ -378,17 +386,6 @@ The parameter to the function specifies the number of [=frames=].
 It must be a positive integer greater than 1.
 
 
-The &lt;single-timing-function&gt; production {#single-timing-function-production}
-=============================================
-
-The syntax for specifying a [=timing function=] is as follows:
-
-<div class="prod"><dfn type>&lt;single-timing-function&gt;</dfn> =
-  ''linear'' |
-  <<cubic-bezier-timing-function>> |
-  <<step-timing-function>> |
-  <<frames-timing-function>></div>
-
 Serialization {#serializing-a-timing-function}
 -------------