Fix service "reset_heating_power"

[saladpanda]

Motivation:

Calling service: better_thermostat.reset_heating_power failed with the following debug log entry:

2023-12-06 22:47:02.206 DEBUG (MainThread) [custom_components.better_thermostat.climate] Service call: <entity climate.buro_heizung_betterthermostat=heat> » reset_heating_power
  File "/config/custom_components/better_thermostat/climate.py", line 113, in async_service_handler
  File "/config/custom_components/better_thermostat/climate.py", line 113, in async_service_handler

Changes:

Added missing parentheses to function call.

Related issue (check one):

• fixes Resetting heating power via service throws error & doesn't work #939
• there is no related issue ticket

Checklist (check one):

• I did not change any code (e.g. documentation changes)
• The code change is tested and works locally.

Test-Hardware list (for code changes)

HA Version: 2023.11.0
Zigbee2MQTT Version:
TRV Hardware: