docs: fix hljs examples breaking

- pin `dgeni-packages` to `0.27.5` as `0.28.0` includes a breaking change
  - https://github.com/angular/dgeni-packages/blob/master/CHANGELOG.md#0280-12-july-2019
- pin `mkdirp` to `0.5.1` to be compatible with `[email protected]`
  - angular/dgeni-packages#287

Author: Splaktar

package-lock.json

@@ -50,7 +50,7 @@
     "colors": "^1.4.0",
     "conventional-changelog": "^3.1.18",
     "dgeni": "^0.4.12",
-    "dgeni-packages": "^0.28.3",
+    "dgeni-packages": "0.27.5",
     "eslint": "^6.8.0",
     "github-contributors-list": "^1.2.4",
     "glob": "^7.1.6",
@@ -86,7 +86,7 @@
     "lazypipe": "^1.0.2",
     "lodash": "^4.17.15",
     "minimist": "^1.2.3",
-    "mkdirp": "^1.0.3",
+    "mkdirp": "0.5.1",
     "moment": "^2.24.0",
     "natives": "^1.1.6",
     "postcss": "^7.0.27",
@@ -120,4 +120,4 @@
     "node": ">=8.17 <12",
     "npm": ">=5"
   }
-}
+}