Revert "doc: remove non-prime from list of primes in spec"

This reverts commit 4b06d9d7.

Reason for revert: It's a reference to a legendary article
from the Journal of Irreproducible Results.

Updates golang/go#24451

Change-Id: I0288177f4e286bd6ace5774f2e5e0acb02370305
Reviewed-on: https://go-review.googlesource.com/101495Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go_spec.html

@@ -2494,7 +2494,7 @@ Examples of valid array, slice, and map literals:
 
 <pre>
 // list of prime numbers
-primes := []int{2, 3, 5, 7, 2147483647}
+primes := []int{2, 3, 5, 7, 9, 2147483647}
 
 // vowels[ch] is true if ch is a vowel
 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}