fix README.md typo (#31)

Correct the word from `plain` to `plan`

Author: GimmyHchs

readme.org

@@ -15,7 +15,7 @@
 
   This is a http cache plugin for caddy 2. In fact, I am not so familiar with the part of cdn cache's mechanism so I reference the code from https://github.com/nicolasazrak/caddy-cache. which is the caddy v1's cache module. Then, I migrate the codebase to be consist with the caddy2 architecture and develop more features based on that.
 
-  Currently, this doesn't support =distributed cache= yet. It's still in plain.
+  Currently, this doesn't support =distributed cache= yet. It's still in plan.
 
 * Features