Fix --fps type conversion to int

[ShinapriLN]

Motivation

I'm using a variety of frame rates, and it's inconvenient to change the value in default_config each time.

Proposed changes

Fixed type conversion for the --fps argument as an integer to improve convenience.

Test

Code:
Ran the command manimgl example.py SceneName --fps 240

Result:
The fps value is now passed as an integer without any errors, making it easier to adjust frame rates dynamically.