fix: Don't crash when mining a landing pad

[Marwes]

This should be landing_pads, not landings_pads (like line 135)

The mod Lunar Landings (1.0.8) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event LunarLandings::on_entity_destroyed (ID 160)
__LunarLandings__/scripts/landing-pad.lua:139: attempt to index field 'landings_pads' (a nil value)
stack traceback:
    __LunarLandings__/scripts/landing-pad.lua:139: in function 'handler'
    __core__/lualib/event_handler.lua:47: in function <__core__/lualib/event_handler.lua:45>

[tburrows13]

Thanks!