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

Documentation/admin-guide/perf-security.rst

@@ -72,7 +72,7 @@ monitoring and observability operations, thus, bypass *scope* permissions
 checks in the kernel. CAP_PERFMON implements the principle of least
 privilege [13]_ (POSIX 1003.1e: 2.2.2.39) for performance monitoring and
 observability operations in the kernel and provides a secure approach to
-perfomance monitoring and observability in the system.
+performance monitoring and observability in the system.
 
 For backward compatibility reasons the access to perf_events monitoring and
 observability operations is also open for CAP_SYS_ADMIN privileged