docs: update Windows cert import instructions
docs: update Windows cert import instructions

as per https://github.com/mitmproxy/mitmproxy/issues/3622#issuecomment-561968523
vhaupert/mitmproxy,mitmproxy/mitmproxy,vhaupert/mitmproxy,mhils/mitmproxy,mhils/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,Kriechi/mitmproxy,vhaupert/mitmproxy,Kriechi/mitmproxy,mhils/mitmproxy,vhaupert/mitmproxy,Kriechi/mitmproxy,mhils/mitmproxy,mhils/mitmproxy,mitmproxy/mitmproxy,mitmproxy/mitmproxy,mitmproxy/mitmproxy