doc: Add ci prefix to CONTRIBUTING.md

Author: hebasto

CONTRIBUTING.md

@@ -88,10 +88,10 @@ the pull request affects. Valid areas as:
   - `refactor` for structural changes that do not change behavior
   - `rpc`, `rest` or `zmq` for changes to the RPC, REST or ZMQ APIs
   - `script` for changes to the scripts and tools
-  - `test` for changes to the bitcoin unit tests or QA tests
+  - `test`, `qa` or `ci` for changes to the unit tests, QA tests or CI code
   - `util` or `lib` for changes to the utils or libraries
   - `wallet` for changes to the wallet code
-  - `build` for changes to the GNU Autotools, reproducible builds or CI code
+  - `build` for changes to the GNU Autotools or reproducible builds
 
 Examples: