doc/admin-guide: fix spelling mistake: "perfomance" -> "performance"

There is a spelling mistake in the perf-security documentation. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210210115624.53551-1-colin.king@canonical.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>