Fix title case to sentence case in "Add Todo"

## What does this MR do?
Implements sentence case for `Add Todo`

## Are there points in the code the reviewer needs to double check?
I changed the images used in the file `doc/workflow/todos.md` to show the new look.

## Why was this MR needed?
#24748 

## Screenshots (if relevant)
Before
![Screen_Shot_2016-11-21_at_12.03.21_PM](/uploads/225abe2901aacccea697596853bdc52c/Screen_Shot_2016-11-21_at_12.03.21_PM.png)
![Screen_Shot_2016-11-21_at_11.58.17_AM](/uploads/3a5997eb6bb037f1c581d9925fa288ce/Screen_Shot_2016-11-21_at_11.58.17_AM.png)

After
![Screen_Shot_2016-11-21_at_12.03.38_PM](/uploads/3be4d58e5adaa10fab759f8726a4d257/Screen_Shot_2016-11-21_at_12.03.38_PM.png)
![Screen_Shot_2016-11-21_at_11.58.37_AM](/uploads/016cdb32cb1e87e9e3cfd89309b53a4a/Screen_Shot_2016-11-21_at_11.58.37_AM.png)


## Does this MR meet the acceptance criteria?

- [X] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [X] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [X] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#24748

See merge request !7724