Copy edit extend-maintenance-mode.txt

Note: For guidelines and instructions pages linked below, log in with user name / password: drupal/drupal.

Task for this issue

Copy edit the following topic:

Title: Enabling and Disabling Maintenance Mode
Section: Extending and Customizing Your Site
File name: extend-maintenance-mode.txt

Look for:
- Typographical errors
- Spelling errors
- Grammar and usage errors
- Punctuation problems
- Adherence to the project's editing guidelines: https://userguide_new-drupal.dev.devdrupal.org/guidelines/guidelines-edi... (access: drupal / drupal )
- Adherence to the project's writing guidelines: https://userguide_new-drupal.dev.devdrupal.org/guidelines/guidelines-wri... (access: drupal / drupal )
- Formatting problems in the output. The User guide output can be found on: https://userguide_new-drupal.dev.devdrupal.org/d8guide/en/index.html (access: drupal / drupal )
- Also note (but do not fix) problems with screen shots in the output. [Making new screenshots is out of scope for this issue. Checking the "alt" text for screenshots is within the scope of this issue, however. So is fixing or at least noting formatting problems with images, such as images that are too expanded, or (conversely) are going out of the page area. See also: https://userguide_new-drupal.dev.devdrupal.org/guidelines/formatting.htm... ]

Note that the "single-issue" guidelines in the Editing guidelines page, as well as the entire Writing guidelines page, should have already been enforced. But it is still a good idea to at least check them quickly.

Instructions

See https://userguide_new-drupal.dev.devdrupal.org/guidelines/instructions.h... -- but skip the section on finding a task to do, since you have selected this one.

Checklist for this task

- Is a patch file provided? If only one or a few topics were edited, you can instead attach the updated source file(s).
- Is the edit complete and correct?
- Is the work free from additional edits outside the scope of the provided guidelines or task?
- Is attribution provided? See https://userguide_new-drupal.dev.devdrupal.org/guidelines/guidelines.htm...