Update nrpe.cfg.j2

[sernafa]

correction to avoid filling the parameter with an incorrect string:
server_address = [u'0.0.0.0 ']

[jloh]

Thanks for this! Unfortunately this will break any running instances for people that are using it as a string currently since the join will split it. It doesn't look like its possible to turn a string into a list natively in jinja2 according to pallets/jinja#705