adds trailing comma to single element tuple
example:
```carbon
package ExplorerTest api;
fn Main() -> i32
{
var a: (i32,) = (1, 2);
return 0;
}
```
change in error message:
```diff
-type error in initializer of variable: '(i32, i32)' is not implicitly convertible to '(i32)'
+type error in initializer of variable: '(i32, i32)' is not implicitly convertible to '(i32,)'
```
Manmeet Singh
f310316ef4