collate/build: documentation fix

Fixes golang/go#19626.

Change-Id: I5801d18b435ff115a2cf3ae5b21a4917afbe4625
Reviewed-on: https://go-review.googlesource.com/44378
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/collate/build/builder.go b/collate/build/builder.go
index 8b5391d..1104284 100644
--- a/collate/build/builder.go
+++ b/collate/build/builder.go
@@ -472,7 +472,6 @@
 }
 
 // Build builds the root Collator.
-// TODO: return Weighter instead
 func (b *Builder) Build() (colltab.Weighter, error) {
 	table, err := b.build()
 	if err != nil {