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

Fixes golang/go#24451

Change-Id: Id9b4cbd1a1ff032f1cc4606e9734ddcc64892ae5
Reviewed-on: https://go-review.googlesource.com/101457Reviewed-by: Filippo Valsorda <filippo@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, 9, 2147483647}
+primes := []int{2, 3, 5, 7, 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}