Update deprecated array access syntax in Porter stemmer

Summary: Fixes T13472. This library uses $a{0}, but this is deprecated in favor of $a[0].

Test Plan:
Ran bin/search index Txxx --force on a task with "filing" in the title (this term reaches the "m" rule of the stemmer).

(I'm not on new enough PHP for this to actually raise an error, but I'll follow up with the reporting user.)

Maniphest Tasks: T13472

Differential Revision: https://secure.phabricator.com/D20941