_content/tour: add missing code tag to var keyword

Through the tour, keywords are consistently marked with backticks,
even in phrasal sentences like "a `return` statement". Add missing
backticks to a var keyword in the basics chapter.

Fixes golang/tour#1764

Change-Id: Ie3d8e2a5135e1a1816061d21c48dea69efe2d6b1
Reviewed-on: https://go-review.googlesource.com/c/website/+/817580
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

_content/tour/basics.article

diff --git a/_content/tour/basics.article b/_content/tour/basics.article
index 9fe0d22..9011382 100644
--- a/_content/tour/basics.article
+++ b/_content/tour/basics.article
@@ -102,7 +102,7 @@
 
 * Variables with initializers
 
-A var declaration can include initializers, one per variable.
+A `var` declaration can include initializers, one per variable.
 
 If an initializer is present, the type can be omitted; the variable will take the type of the initializer.