Remove double negative

doc/administration/operations/fast_ssh_key_lookup.md

@@ -56,7 +56,7 @@ new one, and attempting to pull a repo.
 > **Warning:** Do not disable writes until SSH is confirmed to be working
 perfectly, because the file will quickly become out-of-date.
-In the case of lookup failures (which are not uncommon), the `authorized_keys`
+In the case of lookup failures (which are common), the `authorized_keys`
 file will still be scanned. So git SSH performance will still be slow for many
 users as long as a large file exists.