Grammar fixes + security note

I think it's important to mention the issue with Blockchain.info wallet backup e-mails --- folks need to realise that they must extend good security to their e-mail accounts, and not doing so has been the cause of a few thefts in the past --- but it's a bit of a struggle to keep the text appropriately brief.

Also, couldn't find the right place to edit it and even if I could I'm uncertain, but I thought Electrum would be more appropriately described with checkfaildecentralizecentralizedtxt than checkpassdecentralizespvtxt since Electrum's network is not the same as the Bitcoin one.  Worth checking with someone who understands Electrum better.