Fix linting errors in issue_weight.md

Fix linting errors in https://docs.gitlab.com/ee/user/project/issues/issue_weight.html

Related: #284446 (closed)

14:42  error  Avoid using future tense: "will cost". Use present tense instead.    gitlab.FutureTense
23:12  error  Avoid using future tense: "will appear". 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 by Marcin Sedlak-Jakubowski