cmd/vet: fix doc typo.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/7061050
diff --git a/src/cmd/vet/doc.go b/src/cmd/vet/doc.go
index d19a42f..265575a 100644
--- a/src/cmd/vet/doc.go
+++ b/src/cmd/vet/doc.go
@@ -45,7 +45,7 @@
 
 4. Untagged composite literals, flag -composites
 
-Composite struct literals that do not used the type-tagged syntax.
+Composite struct literals that do not use the type-tagged syntax.
 
 
 Usage: