Update build-docs.yml

Update apt repositories before installing deps

.github/workflows/build-docs.yml

@@ -13,7 +13,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-
+
+    - name: Update apt repositories 
+      run: sudo apt update
+
     - name: Install Doxygen & CMake & ninja
       run: |
         sudo apt install doxygen cmake ninja-build