Improve wording of the web hooks page on the project's setting
- Dev: https://dev.gitlab.org/gitlab/gitlabhq/issues/1947
- Related to https://dev.gitlab.org/gitlab/gitlabhq/issues/1828
The wording on the web hooks page should be improved in order to avoid confusion.
We need to make sure the text is the same as in the documentation so instead of This url will be triggered when a merge request is created
=> This url will be triggered when a merge request is created/updated/merged
cc/ @patricio