Bump docfx to 2.78.2

Author: AArnott

.config/dotnet-tools.json

@@ -24,11 +24,11 @@
       "rollForward": false
     },
     "docfx": {
-      "version": "2.78.1",
+      "version": "2.78.2",
       "commands": [
         "docfx"
       ],
       "rollForward": false
     }
   }
-}
+}