context: Fix removeChild function.

Change-Id: Iae0d109128f7d3214cb4d6f6cf5398068ed9013a
diff --git a/context/context.go b/context/context.go
index f70d2f1..b52ecfc 100644
--- a/context/context.go
+++ b/context/context.go
@@ -246,7 +246,7 @@
 
 func (c *cancelState) removeChild(child *cancelState) {
 	c.mu.Lock()
-	delete(c.children, c)
+	delete(c.children, child)
 	c.mu.Unlock()
 }