content/9years: change Q2 2018 to mid-2018

Per comment from Rob on golang.org/cl/148883.

Change-Id: I8930368db2478b4d750c6060553df01772fae4da
Reviewed-on: https://go-review.googlesource.com/c/148887
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/content/9years.article b/content/9years.article
index ca7fd1d..edc8199 100644
--- a/content/9years.article
+++ b/content/9years.article
@@ -81,7 +81,7 @@
 * Go Contributors
 
 The Go project has been increasing in the number of contributions from the community for several years.
-The project hit a major milestone in Q2 2018 when, for the first time,
+The project hit a major milestone in mid-2018 when, for the first time,
 we had more contributions coming from the community than the Go team.
 
 .image 9years-graph.png _ 600