Correct pasto in 14.7.3 example

Roger Orr asked me to explain how the specialization of A<char>::S could be ill-formed with an int underlying type; the answer is that the underlying type was a copy/paste error in the example.  This patch fixes that error.

This change seems to be on the border of editorial and technical; my guess was that since I wrote the example in the first place I can correct an obvious error, but I'm happy to take it to Core if that seems more appropriate to you.