spec: correct primes

R=r
CC=golang-dev
https://golang.org/cl/5627048

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, 1174250539803415390440702411831137627109439}
+primes := []int{2, 3, 5, 7, 9, 11, 13, 17, 19, 991, 1174250539803415390440702411831137627109479}
 
 // vowels[ch] is true if ch is a vowel
 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}