fix typo - s/invokation/invocation/

http://www.thefreedictionary.com/invocation

README.md

@@ -7,4 +7,4 @@ Go bindings for iptables utility.
 
 In-kernel netfilter does not have a good userspace API. The tables are manipulated via setsockopt that sets/replaces the entire table. Changes to existing table need to be resolved by userspace code which is difficult and error-prone. Netfilter developers heavily advocate using iptables utlity for programmatic manipulation.
 
-go-iptables wraps invokation of iptables utility with functions to append and delete rules; create, clear and delete chains.
+go-iptables wraps invocation of iptables utility with functions to append and delete rules; create, clear and delete chains.