Bump docfx from 2.78.2 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

@@ -24,7 +24,7 @@
       "rollForward": false
     },
     "docfx": {
-      "version": "2.78.2",
+      "version": "2.78.4",
       "commands": [
         "docfx"
       ],