Outdated recceiver start instructions

[minijackson]

I think the recceiver README has outdated instructions on how to start the server with twistd.

From what I understand this line sets up the poll reactor, which means passing -r poll initializes the poll reactor twice, leading to the cryptic error: The specified reactor cannot be used, failed with error: reactor already installed. (see also #93 & #37).

I noticed it while trying to upgrade to recceiver 1.7, my integration test was failing.
The -r poll was needed for recceiver 1.6, but makes it crash at start for recceiver 1.7.

I figured I should remove it by looking at the arguments provided in the corresponding Dockerfile.

[tynanford]

Yes you are right. With 1.7 we also had to remove the -r poll option to recceiver