[C] Fix PlayerToggleFlightEvent

The Issue:

PlayerToggleFlightEvent.isFlying() returns the incorrect value

Justification for this PR:

Breaks plugins using the event PlayerToggleFlightEvent

PR Breakdown:

Currently uses .f()
Which returns canInstantlyBuild boolean

Needs to be changed to .d()
Which returns isFlying

Testing Results and Materials:

I compiled CraftBukkit with the change and tested it with successful results.

JIRA Ticket:

BUKKIT-5172 - https://bukkit.atlassian.net/browse/BUKKIT-5172