content: describe len() when introducing slices

len() is used in subsequent slides to explain other concepts.

Fixes golang/go#12758

Change-Id: I7fd4c48bc84aa2b6d57ea4c1e24149fd3f942bca
Reviewed-on: https://go-review.googlesource.com/15559
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed