Fix linting errors in milestones/index.md

Fix linting errors in https://docs.gitlab.com/ee/user/project/milestones/index.html

Related: #284446 (closed)

50:57    error  Avoid using future tense: "you'll see". Use present tense instead.   gitlab.FutureTense
52:59    error  Avoid using future tense: "you'll only". Use present tense instead.  gitlab.FutureTense
98:102   error  Avoid using future tense: "will merge". Use present tense instead.   gitlab.FutureTense
98:313   error  Avoid using future tense: "will now". Use present tense instead.     gitlab.FutureTense
101:266  error  Avoid using future tense: "will lose". 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.