Incorrect documentation for gp_lttb

[WireBaron]

Describe the bug
Documentation for gp_lttb lists the arguments as (time, value, resolution, then optional gapsize), but the actual implementation is (time, value, optional gapsize, then resolution). Given that this isn't a valid postgresQL function definition (the actual implementation using multiple functions to achieve this), the fix here is probably changing the code to match the documentation rather than the other way around.