Capitalize "OPcache" more consistently

Summary: Fixes T13174. PHP spells this "OPcache" (lowercase "c"); we're inconsistent. Be more consistent.

Test Plan:

• git grep OPCache
• git grep -i opcache | grep -v opcache | grep -v OPcache

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13174

Differential Revision: https://secure.phabricator.com/D19538