Remove duplicated range from regex

`/\w/` is equivalent to `[a-zA-Z0-9_]`. See
http://ruby-doc.org/core-2.2.0/doc/regexp_rdoc.html#label-Character+Classes.

lib/jekyll/site.rb

@@ -528,7 +528,7 @@ module Jekyll
     end
 
     def sanitize_filename(name)
-      name.gsub!(/[^\w\s_-]+/, '')
+      name.gsub!(/[^\w\s-]+/, '')
       name.gsub!(/(^|\b\s)\s+($|\s?\b)/, '\\1\\2')
       name.gsub(/\s+/, '_')
     end