docs: Fix file extension in FrankenPHP configuration (#2067)

Corrected the file extension for additional configuration files in
FrankenPHP section.

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Signed-off-by: Francis Lavoie <lavofr@gmail.com>

Author: francislavoie

docs/config.md

@@ -53,7 +53,7 @@ RUN cp $PHP_INI_DIR/php.ini-development $PHP_INI_DIR/php.ini
 FrankenPHP:
 
 - `/etc/frankenphp/Caddyfile`: the main configuration file
-- `/etc/frankenphp/caddy.d/*.caddy`: additional configuration files that are loaded automatically
+- `/etc/frankenphp/Caddyfile.d/*.caddyfile`: additional configuration files that are loaded automatically
 
 PHP: