Update files table header

[codecovdesign]

Problem to solve

The table in the PR comment that is displayed shows only in the case there are files changed that have reduced coverage. This is not explicit with the current labeling.

Solution

Updated table's File table header to Files with missing lines

[nora-codecov]

some feedback from a codecov connoisseur (@giovanni-guidini), this wording might not be a good substitute. He recommends Files with missing lines instead, what do we think of this @codecovdesign ?

link to Gio's full comment

[codecovdesign]

recommends Files with missing lines

@nora-codecov @giovanni-guidini love it! updated the description accordingly