docs: ABI: testing: ima_policy: Fixed missing bracket

This fixes a minor typo.

Fixes: 34433332 ("docs: ABI: testing: make the files compatible with ReST output")
Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>
Link: https://lore.kernel.org/r/20210215102031.10622-1-michael.weiss@aisec.fraunhofer.de


Signed-off-by: Jonathan Corbet <corbet@lwn.net>