sbox: fix typo when comparing files

A typo caused one of the two files to be compared to itself if they
both had the same size.

Bug: 228496289
Test: Test_filesHaveSameContents
Change-Id: Ie44f32e3c9b8ef725f0d9933c7e701340036710a

cmd/sbox/sbox.go

@@ -750,7 +750,7 @@ func filesHaveSameContents(a, b string) bool {
 		return false
 	}
 	defer fileA.Close()
-	fileB, err := os.Open(a)
+	fileB, err := os.Open(b)
 	if err != nil {
 		return false
 	}