chore(AIP-156): fix minor typo

Author: gnossen

aip/general/0156.md

@@ -57,7 +57,7 @@ rpc UpdateConfig(UpdateConfigRequest) returns (Config) {
   - Example: `users/1234/config`
 - Singleton resources are always singular.
   - Example: `users/1234/thing`
-- Singleton resource defintions **must** provide both the `singular` and
+- Singleton resource definitions **must** provide both the `singular` and
   `plural` fields (see above example).
 - Singleton resources **may** parent other resources.
 - Singleton resources **must not** define the [`Create`][aip-133] or