Accessibility: Fixed label for close button in conversations compose dialog

Fixes CNVS-14516

Test plan:
- Open a conversation
- Open the reply dialog.
- Using a screen reader, verify that the close button is properly read.

Change-Id: I4b5a01c4b2bdc8311d001938a90d1b195a03492a
Reviewed-on: https://gerrit.instructure.com/38888
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>