docs: escape circumflex in sysctl.8

Groff says special characters should be escaped and some gettext
versions choke on it, so escape the ^

References:
 procps-ng/procps#147

sysctl.8

@@ -149,7 +149,7 @@ file that is ordered later).
 .br
 /sbin/sysctl \-a \-\-pattern 'net.ipv4.conf.(eth|wlan)0.arp'
 .br
-/sbin/sysctl \-\-system \-\-pattern '^net.ipv6'
+/sbin/sysctl \-\-system \-\-pattern '\[char94]net.ipv6'
 .SH DEPRECATED PARAMETERS
 The
 .B base_reachable_time