content: revise description of map creation

Change-Id: If81d061de18fd3a8a190c04d3232e8bc247277e8
Reviewed-on: https://go-review.googlesource.com/21449
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Ralph Corderoy <ralph@inputplus.co.uk>
1 file changed