Fix typo: Paypal -> PayPal

Spotted by Siebrand
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Paypal_gateway-desc/en

Change-Id: I3f0522d8f4ece7fe72242d3694dbafcf2c3cfc5c
Merge "Fix typo: Paypal -> PayPal"