Update main.yml (#395)

Update actions/checkout to version 3

.github/workflows/main.yml

@@ -26,7 +26,7 @@ jobs:
 
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v3
 
       - name: Generate Doxyfile
         # Replace CMake's ${ENV_VAR} to Doxygen's $(ENV_VAR) syntax