Fix missing headings in Python 2.7 docs

Author: Thibaut

lib/docs/filters/python/clean_html.rb

@@ -15,7 +15,7 @@ def root
 
       def other
         css('h1').each do |node|
-          node.content = node.content.sub!(/\A[\d\.]+/) do |str|
+          node.content = node.content.sub(/\A[\d\.]+/) do |str|
             rgx = /\A#{str}/
             @levelRegexp = @levelRegexp ? Regexp.union(@levelRegexp, rgx) : rgx
             ''