Update dependency docfx to v2.78.4

Author: renovate[bot]

.config/dotnet-tools.json

@@ -24,7 +24,7 @@
       "rollForward": false
     },
     "docfx": {
-      "version": "2.78.3",
+      "version": "2.78.4",
       "commands": [
         "docfx"
       ],