unsafe: fix reference to string header

Fixes #24115

Change-Id: I89d3d5a9c0916fd2e21fe5930549c4129de8ab48
Reviewed-on: https://go-review.googlesource.com/96983Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/unsafe/unsafe.go

@@ -158,7 +158,7 @@ type ArbitraryType int
 //	hdr.Len = n
 //
 // In this usage hdr.Data is really an alternate way to refer to the underlying
-// pointer in the slice header, not a uintptr variable itself.
+// pointer in the string header, not a uintptr variable itself.
 //
 // In general, reflect.SliceHeader and reflect.StringHeader should be used
 // only as *reflect.SliceHeader and *reflect.StringHeader pointing at actual