Bug 1612988 - Fix warning in a peek_poke test. r=jrmuizel

The test is meant to check that tupes work and a tuple of a single element needs a comma to differentiate it from just being the element.

Differential Revision: https://phabricator.services.mozilla.com/D61681

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/f020ddf9062127a044ea3c026717e33e346ca8bc

peek-poke/tests/max_size.rs

@@ -52,7 +52,7 @@ fn test_fixed_size_array() {
 
 #[test]
 fn test_tuple() {
-    assert_eq!(<(isize)>::max_size(), size_of::<isize>());
+    assert_eq!(<(isize,)>::max_size(), size_of::<isize>());
     assert_eq!(<(isize, isize, isize)>::max_size(), 3 * size_of::<isize>());
     assert_eq!(<(isize, ())>::max_size(), size_of::<isize>());
 }