qa/tasks/thrashosds-health.yaml: whitelist 'slow request'

#25824 adds slow request to OSD logs.
To deal with it, whitelist 'slow request' instead of 'slow requests'.
This PR is specific to luminous because later versions whitelist it correctly.

Signed-off-by: Neha Ojha <nojha@redhat.com>

qa/tasks/thrashosds-health.yaml

@@ -12,4 +12,4 @@ overrides:
       - \(REQUEST_SLOW\)
       - \(TOO_FEW_PGS\)
       - \(MON_DOWN\)
-      - slow requests
+      - slow request