Fix incorrect help information on installing plugins

[majormoses]

Reported in sensu-plugins/sensu-plugins-process-checks#58 it appears that we should be telling people to use -(p|P) && -(e|E) to install plugins and extensions respectivley.

I see two ways that unfortunately mean lots of legwork. We need to get a list of all non deprecated plugins and extensions and either correct them all there or include a link to here: https://github.com/sensu-plugins/community/blob/master/best_practices/production_deployments/plugins/PINNING_VERSIONS.md I prefer to link all back to one place the reasoning is that it means next time we have a simialr issue we update it once, the other reason is that people should be aware of versionin pinning.

[majormoses]

Before anyone starts running off we should come to an agreement on how to do it to keep it consistent.