doc/effective_go.html: fix typo

Fixes #5296.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8682044

doc/effective_go.html

@@ -2555,7 +2555,7 @@ if _, ok := val.(json.Marshaler); ok {
 
 <p>
 One place this situation arises is when it is necessary to guarantee within the package implementing the type that
-it it actually satisfies the interface.
+it actually satisfies the interface.
 If a type—for example,
 <code><a href="/pkg/encoding/json/#RawMessage">json.RawMessage</a></code>—needs
 a custom its JSON representation, it should implement