Fix malformed reST (#1802)

Commit 2b045757 ("Improve documentation related to inheritance. (#1676)") left off
a ':' from a hyperlink, which breaks the Travis CI build.

docs/advanced/classes.rst

@@ -325,7 +325,7 @@ can now create a python class that inherits from ``Dog``:
 Extended trampoline class functionality
 Extended trampoline class functionality
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-.. _extended_class_functionality_forced_trampoline
+.. _extended_class_functionality_forced_trampoline:
 
 
 Forced trampoline class initialisation
 Forced trampoline class initialisation
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