Fix small typo in coverage.md

[iustin]

A missing closing brace left the text being show as: "… to be rebuilt (see #1940."

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[mattaudesse]

Thanks @iustin!