fix typo redcarpet requirement in lib/redmine.rb (#15520)

git-svn-id: http://svn.redmine.org/redmine/trunk@12461 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/redmine.rb

@@ -23,7 +23,7 @@ rescue LoadError
   # RMagick is not available
 end
 begin
-  require 'Redcarpet' unless Object.const_defined?(:Redcarpet)
+  require 'redcarpet' unless Object.const_defined?(:Redcarpet)
 rescue LoadError
   # Redcarpet is not available
 end