spec: 9 only looks prime

Fixes #2870.

R=golang-dev, agl, lvd
CC=golang-dev
https://golang.org/cl/5618050

doc/go_spec.html

@@ -2143,7 +2143,7 @@ Examples of valid array, slice, and map literals:
 
 <pre>
 // list of prime numbers
-primes := []int{2, 3, 5, 7, 9, 11, 13, 17, 19, 991}
+primes := []int{2, 3, 5, 7, 11, 13, 17, 19, 991}
 
 // vowels[ch] is true if ch is a vowel
 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}