Fix call-seq of Encoding::Converter#putback [ci skip]

[Fix GH-1999]

From: Takayuki Matsubara <takayuki.1229@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65487 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

transcode.c

@@ -4084,7 +4084,7 @@ econv_insert_output(VALUE self, VALUE string)
 }
 
 /*
- * call-seq
+ * call-seq:
  *   ec.putback                    -> string
  *   ec.putback(max_numbytes)      -> string
  *