Fix part of #5699: Enable workflow dispatching for wiki jobs (#5703)

## Explanation

Fixes part of #5699 (even though it's already expected to be fixed).

This was missed in #5700. We can't verify the fixes from that PR without
the ability to re-run the workflow outside of a PR (or until another PR
that modifies the wiki is merged). It seems ideal to have the ability to
force-refresh the wiki in this way, anyway.

Similar to #5700, this can't be easily verified until it's merged.

## Essential Checklist
- [x] The PR title and explanation each start with "Fix #bugnum: " (If
this PR fixes part of an issue, prefix the title with "Fix part of
#bugnum: ...".)
- [x] Any changes to
[scripts/assets](https://github.com/oppia/oppia-android/tree/develop/scripts/assets)
files have their rationale included in the PR explanation.
- [x] The PR follows the [style
guide](https://github.com/oppia/oppia-android/wiki/Coding-style-guide).
- [x] The PR does not contain any unnecessary code changes from Android
Studio
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#undo-unnecessary-changes)).
- [x] The PR is made from a branch that's **not** called "develop" and
is up-to-date with "develop".
- [x] The PR is **assigned** to the appropriate reviewers
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#clarification-regarding-assignees-and-reviewers-section)).

## For UI-specific PRs only
N/A -- This only affects GitHub actions wiki workflows.

.github/workflows/wiki.yml

@@ -1,6 +1,7 @@
 name: Deploy to Wiki
 
 on:
+  workflow_dispatch:
   pull_request:
     paths:
       - 'wiki/**'