Fix typo [ci skip]

* doc/extension.rdoc: [DOC] fix double-word typo.  [Fix GH-1153]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53133 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,7 @@
+Wed Dec 16 00:39:27 2015  Jake Worth  <jakeworth82@gmail.com>
+
+	* doc/extension.rdoc: [DOC] fix double-word typo.  [Fix GH-1153]
+
 Wed Dec 16 00:25:41 2015  Naohisa Goto  <ngotogenome@gmail.com>
 
 	* lib/webrick/utils.rb (WEBrick::Utils::TimeoutHandler#initialize):

doc/extension.rdoc

@@ -1767,7 +1767,7 @@ barriers. Please carefully consider the risks.
 
 Before inserting write barriers, you need to know about RGenGC algorithm
 (gc.c will help you). Macros and functions to insert write barriers are
-available in in include/ruby/ruby.h. An example is available in iseq.c.
+available in include/ruby/ruby.h. An example is available in iseq.c.
 
 For a complete guide for RGenGC and write barriers, please refer to
 <https://bugs.ruby-lang.org/projects/ruby-trunk/wiki/RGenGC>.