content: fix grammar in nil maps slide

Change-Id: I7f4bf5ad97aed66b59ee9087241a71e3beee40cb
Reviewed-on: https://go-review.googlesource.com/21590
Reviewed-by: Rob Pike <r@golang.org>

content/moretypes.article

diff --git a/content/moretypes.article b/content/moretypes.article
index 9cac743..a410fb5 100644
--- a/content/moretypes.article
+++ b/content/moretypes.article
@@ -195,7 +195,7 @@
 A map maps keys to values.
 
 The zero value of a map is `nil`.
-A `nil` map has no keys, nor can they be added.
+A `nil` map has no keys, nor can keys be added.
 
 The `make` function returns a map of the given type,
 initialized and ready for use.