[x/tour] 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>
X-Tour-Commit: 9d789a4a34a57a0a20e92cc41f525e4e84914f34

tour/content/moretypes.article

@@ -83,6 +83,8 @@ A slice points to an array of values and also includes a length.
 
 `[]T` is a slice with elements of type `T`.
 
+`len(s)` returns the length of slice `s`.
+
 .play moretypes/slices.go
 
 * Slicing slices