doc: fix typo in Effective Go

Fixes #2120.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4810061

doc/effective_go.html

@@ -2730,7 +2730,7 @@ suppresses the usual check for a <code>return</code> statement.
 <pre>
 // A toy implementation of cube root using Newton's method.
 func CubeRoot(x float64) float64 {
-    z := x/3   // Arbitrary intitial value
+    z := x/3   // Arbitrary initial value
     for i := 0; i &lt; 1e6; i++ {
         prevz := z
         z -= (z*z*z-x) / (3*z*z)