unicode/bidi: remove vet complaint

Change-Id: Ia179c919ecafbeeccb916abbcf553fc3a204dcfb
Reviewed-on: https://go-review.googlesource.com/38360
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unicode/bidi/core.go

diff --git a/unicode/bidi/core.go b/unicode/bidi/core.go
index ec52f14..d4c1399 100644
--- a/unicode/bidi/core.go
+++ b/unicode/bidi/core.go
@@ -726,7 +726,7 @@
 				continue loop
 			}
 		}
-		log.Panicf("invalid bidi code %s present in assertOnly at position %d", t, s.indexes[i])
+		log.Panicf("invalid bidi code %v present in assertOnly at position %d", t, s.indexes[i])
 	}
 }