Updates to OpenPGP best practices

When translating into spanish i noticed some issues with the OpenPGP Best Practices document. This will remove a wrong link in the document, fix some incosistences as GnuPG is an implementation for OpenPGP and it also will fix some typos. 
yeah, the instructions are wrong. I support just removing the link for now. 

one thing we could do is sign the cert with the riseup key. that way, if a user has followed any of the other pages about verifying stuff that is on help.riseup.net, they will already have the key.