Fix name of @cmd_group.result_callback()

[markvels]

When I tried to run a simple command of tc420, I get an error about a name resolution:

Traceback (most recent call last): File "/home/user/.local/share/virtualenvs/tc420-7Nt0gyJ3/bin/tc420", line 5, in from tc420.main import main File "/home/user/src/tc420/tc420/main.py", line 286, in @cmd_group.resultcallback() ^^^^^^^^^^^^^^^^^^^^^^^^ AttributeError: 'Group' object has no attribute 'resultcallback'. Did you mean: 'result_callback'?

This patch fixes this error by adjusting the name.