* include/ruby/missing.h: fix typo and so on.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@32840 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

include/ruby/missing.h

@@ -52,7 +52,7 @@ struct timezone {
 #endif
 
 #if defined(HAVE___SYSCALL) && (defined(__APPLE__) || defined(__OpenBSD__))
-/* Mac OS X and OpenBSD has __syscall but doen't defined in headers */
+/* Mac OS X and OpenBSD have __syscall but don't define it in headers */
 off_t __syscall(quad_t number, ...);
 #endif