Bump docfx to 2.78.4

Author: roji

.github/workflows/build-documentation.yml

@@ -67,7 +67,7 @@ jobs:
       working-directory: EFCore.PG
 
     - name: Get docfx
-      run: dotnet tool install --version 2.78.3 -g docfx
+      run: dotnet tool install --version 2.78.4 -g docfx
 
     - name: Build docs
       run: docfx --warningsAsErrors