fix brew install doxygen warning

Author: dlebauer

.github/workflows/ci.yml

@@ -31,7 +31,7 @@ jobs:
         if [[ "$RUNNER_OS" == "Linux" ]]; then
           sudo apt-get install doxygen -y
         elif [[ "$RUNNER_OS" == "macOS" ]]; then
-          brew install doxygen
+          brew install --formula doxygen
         fi
         
     # compile SIPNET