[x/blog] go.blog/content/slices: rewrite an ambiguous sentence

Someone was confused by the sentence introducing the relationship
between slice headers and the arrays they describe. Rewrite to eliminate
the ambiguity.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/14680043
X-Blog-Commit: a28f38c30686b995e8853dedb5e295c43c2c2d3d

blog/content/slices.article

@@ -73,7 +73,7 @@ for a slice.
 Slices are where the action is, but to use them well one must understand
 exactly what they are and what they do.
 
-A slice is a data structure that describes a contiguous section of an array, which is
+A slice is a data structure describing a contiguous section of an array
 stored separately from the slice variable itself.
 _A_slice_is_not_an_array_.
 A slice _describes_ a piece of an array.