content: use numbers for post series description

Rather than a mix of words and numbers.

Change-Id: I2102f11f4e66167b9708d5cfb96e2b4a02bc40c1
Reviewed-on: https://go-review.googlesource.com/c/blog/+/191178
Reviewed-by: Jay Conrod <[email protected]>

Author: jeanbza

content/using-go-modules.article

@@ -7,7 +7,7 @@ Tyler Bui-Palsulich
 Eno Compton
 
 * Introduction
-This post is part one in a series. See part 2 — [[https://blog.golang.org/migrating-to-go-modules][Migrating to Go Modules]].
+This post is part 1 in a series. See part 2 — [[https://blog.golang.org/migrating-to-go-modules][Migrating to Go Modules]].
 
 Go 1.11 and 1.12 include preliminary
 [[https://golang.org/doc/go1.11#modules][support for modules]],