Events: Switch PCRE \d token to POSIX [0-9]

The previous code worked in production, but this change is needed for it to also work in the Docker environment.

public_html/wp-content/themes/wporg-events-2023/inc/city-landing-pages.php

@@ -54,7 +54,7 @@ function is_city_landing_page(): bool {
 			path REGEXP %s
 		LIMIT 1",
 		EVENTS_NETWORK_ID,
-		"^/$city/\d{4}/[a-z-]+/$"
+		"^/$city/[0-9]{4}/[a-z-]+/$"
 	) );
 
 	return ! empty( $sites );