Amstrad CPC — add .cpr file extension

[josiang]

Description

This PR adds a missing file extension in the Amstrad CPC emulator config file.

.cpr files are CPC Plus cartridge image and a handful of Amstrad games are preserved using this format.
libretro-cap32 handles those files without issue.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested Locally?

Tested locally by editing the emulation station systems config manually in an already built JELOS.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[fewtarius]

Thanks for the contribution! ❤️