qa: use correct variable for exception debug

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit 096a5ca)

qa/tasks/cephfs/test_strays.py

@@ -248,7 +248,7 @@ def background():
         elif throttle_type == self.FILES_THROTTLE:
             if files_high_water < mds_max_purge_files / 2:
                 raise RuntimeError("Files in flight high water is unexpectedly low ({0} / {1})".format(
-                    ops_high_water, mds_max_purge_files
+                    files_high_water, mds_max_purge_files
                 ))
 
         # Sanity check all MDC stray stats