Fix get_limits documentation

[ryanmelt]

The get_limits documentation is incorrect in stating how it operates.

I have tried this syntax (as listed in docs):
get_limits(<Target Name>, <Packet Name>, <Item Name>, <Limits Set (optional)>)

but I get:
ArgumentError : wrong number of arguments (given 4, expected 3)

The code definition is:
def get_limits(target_name, packet_name, item_name, cache_timeout: nil, manual: false, scope: $openc3_scope, token: $openc3_token)

That definition does not appear to allow for providing the limits set name as an arg or kwarg. The get_limits method returns a hash of all defined limits for that target, packet, item which I can then do this with:
get_limits(<Target Name>, <Packet Name>, <Item Name>)[<Limits Set>]

That gets the particular limits set information I required.