[Doc] Update README to reflect the use of -t option instead of -g option

Author: ydah

README.rdoc

@@ -52,7 +52,7 @@
   Any kind of bug report is welcome.
   If you find a bug of Racc, please report an issue at 
   https://github.com/ruby/racc/issues. Your grammar file,
-  debug output generated by "racc -g", are helpful.
+  debug output generated by "racc -t", are helpful.
 
 
                                                       Minero Aoki