Fix comment in pg_upgrade.h.

Fix comment in pg_upgrade.h.

Contrary to what the comment for the "check" struct member claims,
'pg_upgrade --check' performs only the checks and does not ask the
user for permission to make changes.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/ZnHk7ci5IuTWVc_c%40nathan