header size for windows in readme

Former-commit-id: 089ee6d0f7e8e684c97494d2d1b8292e945a4ad8

README.md

@@ -145,7 +145,7 @@ To build the a static YImg library on Mac OS X issue:
     make
 
 
-## Windows (Experimental) ##
+### Windows (Experimental) ###
 To build a static library (.lib) on windows, open Visual Studio 2010.
 
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