[FAB-6657] peer channel join description update
[FAB-6657] peer channel join description update

This CR updates the description for the `peer channel join` command
to refer to a channel instead of a chain.

Change-Id: I1d157b13430d32d13a9c941b6caa7893dc485042
Signed-off-by: Will Lahti <f19a2779710675aad3df5c81e4ecfde11707e3aa@us.ibm.com>

muralisrini/fabric,ckeyer/fabric,manish-sethi/fabric-sidedb,ckeyer/fabric,lukehuangch/fabric,jimthematrix/fabric,muralisrini/fabric,manish-sethi/fabric,manish-sethi/fabric-sidedb,ckeyer/fabric,ckeyer/fabric,ckeyer/fabric,lukehuangch/fabric,lukehuangch/fabric,binhn/fabric,stemlending/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,xixuejia/fabric,stemlending/fabric,lukehuangch/fabric,hyperledger/fabric,ckeyer/fabric,lukehuangch/fabric,ckeyer/fabric,stemlending/fabric,lukehuangch/fabric,manish-sethi/fabric-sidedb,stemlending/fabric,manish-sethi/fabric,xixuejia/fabric,binhn/fabric,hyperledger/fabric,manish-sethi/fabric-sidedb,binhn/fabric,jimthematrix/fabric,xixuejia/fabric,manish-sethi/fabric-sidedb,muralisrini/fabric,lukehuangch/fabric