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] <[email protected]>

Author: dependabot[bot]

.config/dotnet-tools.json

@@ -3,7 +3,7 @@
   "isRoot": true,
   "tools": {
     "docfx": {
-      "version": "2.78.3",
+      "version": "2.78.4",
       "commands": [
         "docfx"
       ],