go-tour: fix list on Zero Values slide

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/130610044
diff --git a/content/basics.article b/content/basics.article
index a3e345a..1f54048 100644
--- a/content/basics.article
+++ b/content/basics.article
@@ -148,6 +148,7 @@
 _zero_value_.
 
 The zero value is:
+
 - `0` for numeric types,
 - `false` the boolean type, and
 - `""` (the empty string) for strings.