Change label for variance section to just 'variance' (#3429)

This will allow the links in #3411 to be shorter.

Author: quartox

docs/source/common_issues.rst

@@ -180,7 +180,7 @@ not support ``sort()``) as a list and sort it in-place:
         # Type of x is List[int] here.
         x.sort()  # Okay!
 
-.. _invariance-vs-covariance:
+.. _variance
 
 Invariance vs covariance
 ------------------------

docs/source/revision_history.rst

@@ -31,7 +31,7 @@ List of major changes:
 
     * Add :ref:`variance-of-generics`.
 
-    * Add :ref:`invariance-vs-covariance`.
+    * Add :ref:`variance`.
 
     * Updates to :ref:`python-36`.