[web] Fix message search copy

Summary: Fix message search copy, as requested in https://linear.app/comm/issue/ENG-4398/fix-copy-for-web-message-search

Test Plan: Checked that the header of the message search modal now says "Search messages"

Reviewers: ashoat, michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8516