Bump docfx from 2.78.3 to 2.78.4

---
updated-dependencies:
- dependency-name: docfx
  dependency-version: 2.78.4
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.config/dotnet-tools.json

@@ -3,9 +3,11 @@
   "isRoot": true,
   "tools": {
     "docfx": {
-      "version": "2.78.3",
-      "commands": ["docfx"],
+      "version": "2.78.4",
+      "commands": [
+        "docfx"
+      ],
       "rollForward": false
     }
   }
-}
+}