errgroup: fix a typo in the documentation

Change-Id: I265cbc977e15a81c0068e7b60933843d8f02fe6a
Reviewed-on: https://go-review.googlesource.com/c/sync/+/747140
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

errgroup/errgroup_test.go

diff --git a/errgroup/errgroup_test.go b/errgroup/errgroup_test.go
index 85008eb..3b563af 100644
--- a/errgroup/errgroup_test.go
+++ b/errgroup/errgroup_test.go
@@ -34,7 +34,7 @@
 
 // JustErrors illustrates the use of a Group in place of a sync.WaitGroup to
 // simplify goroutine counting and error handling. This example is derived from
-// the sync.WaitGroup example at https://golang.org/pkg/sync/#example_WaitGroup.
+// the sync.WaitGroup example at https://golang.org/pkg/sync/#example-WaitGroup.
 func ExampleGroup_justErrors() {
 	g := new(errgroup.Group)
 	var urls = []string{