Fix typo of rdoc. [ruby-core:35016]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@30882 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

marshal.c

@@ -864,7 +864,7 @@ clear_dump_arg(struct dump_arg *arg)
 
 /*
  * call-seq:
- *      dump( obj [, anIO] , limit=--1 ) -> anIO
+ *      dump( obj [, anIO] , limit=-1 ) -> anIO
  *
  * Serializes obj and all descendant objects. If anIO is
  * specified, the serialized data will be written to it, otherwise the