Replace P2PH With P2PKH

Suggested by @luke-jr in #393 (thanks!), this pull changes the abbreviation for Pay-To-Public-Key from P2PH to P2PKH.  Text and link changes by @saivann; image changes by @harding.

I recommend merging this into #393 as soon as Saïvann checks it over.  (Jekyll compiles cleanly and linkchecker -a reports no broken internal links here.)