Minor initial fix for missing comma (#3943)

Co-authored-by: Peter McNeeley <[email protected]>

src/webgpu/shader/validation/expression/matrix/mul.spec.ts

@@ -631,7 +631,7 @@ g.test('overflow_mat_f16_internal')
     for (let i = 0; i < t.params.c; i++) {
       for (let k = 0; k < t.params.r; k++) {
         lhs += `${t.params.lhs},`;
-        rhs += `1`;
+        rhs += `1,`;
       }
     }
     rhs += ')';