content: ensure better semver versions in the v2 modules article

The Semantic Versioning specification at https://semver.org uses dots to
separate the numeric part of a pre-release version from the non-numeric
to ensure that they sort correctly.  Make the article comply.

Change-Id: I7a366e4c180f5048b1cc19cdf3e66c515989e4ab
Reviewed-on: https://go-review.googlesource.com/c/blog/+/206157
Reviewed-by: Jean de Klerk <deklerk@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
diff --git a/content/v2-go-modules.article b/content/v2-go-modules.article
index 634c3dc..db2c0cd 100644
--- a/content/v2-go-modules.article
+++ b/content/v2-go-modules.article
@@ -150,8 +150,8 @@
 the new API. If we want users to be able to experiment with the new API before
 we officially make it stable, we can publish a `v2` pre-release version:
 
-    $ git tag v2.0.0-alpha1
-    $ git push origin v2.0.0-alpha1
+    $ git tag v2.0.0-alpha.1
+    $ git push origin v2.0.0-alpha.1
     $
 
 Once we are happy with our `v2` API and are sure we don't need any other breaking