content: fix typo in concurrency article

Fixes golang/tour#372

Change-Id: I1b7362304d2e133e1412922252cabb10a7af89ad
Reviewed-on: https://go-review.googlesource.com/121895
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
diff --git a/content/concurrency.article b/content/concurrency.article
index 3d59db5..913c727 100644
--- a/content/concurrency.article
+++ b/content/concurrency.article
@@ -1,5 +1,5 @@
 Concurrency
-Go provides concurrency constructions as part of the core language. This lessons presents them and gives some examples on how to use it.
+Go provides concurrency constructions as part of the core language. This lesson presents them and gives some examples on how they can be used.
 
 The Go Authors
 https://golang.org