US 116-177 21.4.13p9 [meta.reflection.substitute] Clarify error message in example

[wg21bot]

The first error message could be clarified to explicitly say that the undeduced placeholder type is the return type.

Proposed change:

Reword as: // error: fn<int> contains an undeduced placeholder type for its return type.

[jensmaurer]

Accepted.