[C] Fix PlayerToggleFlightEvent

The Issue:

PlayerToggleFlightEvent.isFlying() returns the incorrect value

Justification for this PR:

Breaks plugins using the event PlayerToggleFlightEvent

PR Breakdown:

Currently used .f()
https://github.com/SpigotMC/mc-dev/blob/master/net/minecraft/server/PacketPlayInAbilities.java#L90

Which returns canInstantlyBuild boolean

Needs to be changed to .d()
https://github.com/SpigotMC/mc-dev/blob/master/net/minecraft/server/PacketPlayInAbilities.java#L74

Which returns isFlying

Testing Results and Materials:

I compiled CraftBukkit with the change and tested it with successful results.

JIRA Ticket: (Will get updated in a min)

BUKKIT-#### - https://bukkit.atlassian.net/browse/BUKKIT-####