Fix linting errors in reply_by_email.md

Fix linting errors in https://docs.gitlab.com/ee/administration/reply_by_email.html

29:61  error  Avoid using future tense: "will:". Use present tense instead.      gitlab.FutureTense
39:37  error  Avoid using future tense: "will look". Use present tense instead.  gitlab.FutureTense
45:29  error  Avoid using future tense: "will be". Use present tense instead.    gitlab.FutureTense

If you'd like to work on this issue, mention @msedlakjakubowski in a comment. Then, in your merge request's description, paste this issue's URL so they become linked.

Edited Dec 01, 2020 by Marcin Sedlak-Jakubowski