imports: fix LocalPrefix document prefix

golint suggested:
  comment on exported var LocalPrefix should be of the form "LocalPrefix ..."

Change-Id: I1d2f92ba76e3c091440f676936dbb1fc1ce07f3a
Reviewed-on: https://go-review.googlesource.com/46432
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/imports/fix.go b/imports/fix.go
index e772b6d..f26414a 100644
--- a/imports/fix.go
+++ b/imports/fix.go
@@ -32,7 +32,7 @@
 	testMu  sync.RWMutex // guards globals reset by tests; used only if inTests
 )
 
-// If set, LocalPrefix instructs Process to sort import paths with the given
+// LocalPrefix, if set, instructs Process to sort import paths with the given
 // prefix into another group after 3rd-party packages.
 var LocalPrefix string