Fix the documentation about changing the logging level (#1596)

Authors:
  - tsuki (https://github.com/enp1s0)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #1596

docs/source/developer_guide.md

@@ -255,7 +255,7 @@ There are 7 logging levels with each successive level becoming quieter:
 7. RAFT_LEVEL_OFF
    Pass one of these as per your needs into the `set_level()` method as follows:
 ```cpp
-raft::logger::get.set_level(RAFT_LEVEL_WARN);
+raft::logger::get().set_level(RAFT_LEVEL_WARN);
 // From now onwards, this will print only WARN and above kind of messages
 ```