Improves the proxy certificates howto doc.

The current documentation contains a bunch of spelling and grammar mistakes. I also
found it hard to understand some paragraphs, so here is my attempt to improve its
readability.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Correct some layout issues, convert all remaining tabs to appropriate amounts of spaces.

Reviewed-by: Matt Caswell <matt@openssl.org>