Fix PHP 8.1 Fatal error in Figlet: Use square brackets instead of curly braces

Summary:
Replace curly with square brackets when accessing an array value. See
https://www.php.net/manual/en/migration74.deprecated.php#migration74.deprecated.core.array-string-access-curly-brace
https://wiki.php.net/rfc/deprecate_curly_braces_array_access#wasn_t_the_curly_brace_syntax_deprecated_once_before

Closes T15406

Test Plan:
Fixed these two lines; afterwards Figlet rendered in task comment preview. Example figlet:

figlet {{{ stuff }}}

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15406

Differential Revision: https://we.phorge.it/D25233