refactor: remove deprecated substr

[masterX89]

Hello, String.prototype.substr() is a deprecated function, maybe use String.prototype.slice() is better.

String.prototype.substr() is defined in Annex B of the ECMA-262 standard. We can find introduction like this:

These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code.

[zkochan]

I think .substring() should be fine

[masterX89]

I think .substring() should be fine

changed to .substring()