[docs] Update glob in docs to include .mjs and .cjs

[jakub-g]

The docs are saying:

By default, mocha looks for the glob "./test/*.js"

But this contradicts what is written at the beginning of the docs

Mocha discovers test files; when given no files or directories, it finds files with extensions .js, .mjs or .cjs in the test directory (but not its children), relative to the current working directory

[outsideris]

Good catch. Thank you.