improve English for a warning message

On DOSish systems, there is a warning message for \r\n line endings on shebang line.
Improve this message from "shebang line ends with \r may cause a problem"
to "shebang line ending with \r may cause problems".

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58981 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ruby.c

@@ -1740,7 +1740,7 @@ static void
 warn_cr_in_shebang(const char *str, long len)
 {
     if (str[len-1] == '\n' && str[len-2] == '\r') {
-	rb_warn("shebang line ends with \\r may cause a problem");
+	rb_warn("shebang line ending with \\r may cause problems");
     }
 }
 #else