DOC: lua: fix Proxy.get_mode() output

DOC: lua: fix Proxy.get_mode() output

Proxy.get_mode() function internally relies on proxy_mode_str() to return
the proxy mode. The current function description is exhaustive about the
possible outputs for the function. I can't tell if it's relevant or not
but it's subject to changes. Here it is the case, the documentation
indicates that "health" mode may be returned, which cannot happen
since 77e0daef9 ("MEDIUM: proxy: remove obsolete "mode health"").

This should be backported up to 2.4