arp(8): fixed one more typo (simplyfy -> simplify)

man/en_US/arp.8

@@ -99,7 +99,7 @@ practice, but is supported by older kernels because it can be
 useful. If the
 .B temp
 flag is not supplied entries will be permanent stored into the ARP
-cache. To simplyfy setting up entries for one of your own network interfaces, you can use the
+cache. To simplify setting up entries for one of your own network interfaces, you can use the
 .B "arp \-Ds"
 .I address ifname
 form. In that case the hardware address is taken from the interface with the