cases: update reference to "unicode/precis"

Change-Id: I32f726a791f39c158a782ed013b1d358f0b27e58
Reviewed-on: https://go-review.googlesource.com/39198
Reviewed-by: Nigel Tao <nigeltao@golang.org>
diff --git a/cases/cases.go b/cases/cases.go
index 0204372..736c0c3 100644
--- a/cases/cases.go
+++ b/cases/cases.go
@@ -88,8 +88,8 @@
 //
 // Case folding does not normalize the input and may not preserve a normal form.
 // Use the collate or search package for more convenient and linguistically
-// sound comparisons.  Use unicode/precis for string comparisons where security
-// aspects are a concern.
+// sound comparisons. Use golang.org/x/text/secure/precis for string comparisons
+// where security aspects are a concern.
 func Fold(opts ...Option) Caser {
 	return Caser{makeFold(getOpts(opts...))}
 }