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>
(cherry picked from commit 03b637a730e4a298c360cc143de7564060c06324)
Correct some layout issues, convert all remaining tabs to appropriate amounts of spaces.

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 8123d158ab6f0a6a468748c133e33c2063ff36b5)