[DOCS] Remove todo from dashboard's "Implement new widget" chapter

The todo is obviously outdated, it is therefore removed.

Additionally, remove some superfluous spaces in the code example and
correct the indentation in the JS section to avoid "quote style" as
drive-by.

Resolves: #102119
Releases: main, 12.4
Change-Id: I357d878d456bdd7f5666d6982fd42337b4d2f6fc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81378


Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>