fix pg_upgrade comment

I noticed that the "check" variable, which is used for "pg_upgrade
--check", is commented as follows:

    bool        check;            /* true -> ask user for permission to make
                                 * changes */

This comment was first added when pg_upgrade was introduced (commit
c2e9b2f288), but I imagine it predates even that.  I've attached a patch to
fix this.  Barring objections, I'll probably commit this soon.

-- 
nathan