Fixed #17933 -- Added a missing label to the docs.

docs/intro/whatsnext.txt

@@ -191,6 +191,8 @@ You can get a local copy of the HTML documentation following a few easy steps:
 __ http://sphinx.pocoo.org/
 __ http://www.gnu.org/software/make/
 
+.. _differences-between-doc-versions:
+
 Differences between versions
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