clarify that `--cert` expects a leaf certificate
clarify that `--cert` expects a leaf certificate

refs #2254
MatthewShao/mitmproxy,mitmproxy/mitmproxy,mitmproxy/mitmproxy,vhaupert/mitmproxy,vhaupert/mitmproxy,ujjwal96/mitmproxy,mhils/mitmproxy,Kriechi/mitmproxy,StevenVanAcker/mitmproxy,cortesi/mitmproxy,MatthewShao/mitmproxy,MatthewShao/mitmproxy,cortesi/mitmproxy,mitmproxy/mitmproxy,mhils/mitmproxy,mhils/mitmproxy,ddworken/mitmproxy,mitmproxy/mitmproxy,ddworken/mitmproxy,ddworken/mitmproxy,Kriechi/mitmproxy,zlorb/mitmproxy,zlorb/mitmproxy,cortesi/mitmproxy,vhaupert/mitmproxy,cortesi/mitmproxy,ujjwal96/mitmproxy,ddworken/mitmproxy,vhaupert/mitmproxy,mhils/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,StevenVanAcker/mitmproxy,mhils/mitmproxy,StevenVanAcker/mitmproxy,ujjwal96/mitmproxy,ujjwal96/mitmproxy,Kriechi/mitmproxy,zlorb/mitmproxy,zlorb/mitmproxy,StevenVanAcker/mitmproxy,MatthewShao/mitmproxy