Fix typo in `lib/mruby/presym.rb`

lib/mruby/presym.rb

@@ -99,7 +99,7 @@ module MRuby
     end
 
     def list_path
-      @list_pat ||= "#{@build.build_dir}/presym".freeze
+      @list_path ||= "#{@build.build_dir}/presym".freeze
     end
 
     def header_dir;