Fix typo in README

Author: nakamuraraika

Diff for: README.md

@@ -88,7 +88,7 @@ function that may fail.
 
 ```kotlin
 fun checkPrivileges(user: User, command: Command): Result<Command, CommandError> {
-    return if (user.rank >= command.mininimumRank) {
+    return if (user.rank >= command.minimumRank) {
         Ok(command)
     } else {
         Err(CommandError.InsufficientRank(command.name))