fix printing for single element tuple values (#2786)

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,)'
```

explorer/testdata/interface/fail_impl_bad_member.carbon

@@ -23,8 +23,8 @@ class Point {
     fn Scale[self: Point](v: i32) -> i32 {
       return 0;
     // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_bad_member.carbon:[[@LINE+3]]: type error in member of implementation
-    // CHECK:STDERR: expected: fn (i32) -> class Point
-    // CHECK:STDERR: actual: fn (i32) -> i32
+    // CHECK:STDERR: expected: fn (i32,) -> class Point
+    // CHECK:STDERR: actual: fn (i32,) -> i32
     }
   }
 }