Tweak label on People page Search people field

Fixes CNVS-14027

Test Plan:
1. Be logged in as a teacher
2. Go to the People page
3. Using a screen reader navigate to the Search People box
4. Hear that the screen reader now says what the field is, before
   describing how to use it.

Change-Id: I0db141e47c6e9be150fc7ee89944b79efe42fa38
Reviewed-on: https://gerrit.instructure.com/39011
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Naberezny <mnaberezny@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>