added regex checking for 500px

The criteria for the the 500px usernames was found my looking at the packages when signing up. If it was an invalid username, the request return this error: https://i.imgur.com/JS5PVMz.png

sherlock/resources/data.json

@@ -50,6 +50,7 @@
     "errorMsg": "Oops! This page doesn\u2019t exist.",
     "errorType": "message",
     "rank": 3453,
+    "regexCheck":"^[a-z0-9_]$",
     "url": "https://500px.com/{}",
     "urlMain": "https://500px.com/",
     "username_claimed": "blue",