Do not use "_" in --testTimeout examples

[IanVS]

Jest apparently can't handle it, and it won't apply the setting

What I did

Updated documentation on the --testTimeout parameter, since I found that it was not working as-documented.

• I removed the _ from numbers like 15_000, because they caused the setting to not be applied.
• I also clarified the wording slightly, since this is not really a "default", rather it is a global setting that cannot be overridden per-test (see [Feature Request] Storybook default tests timeouts must accept overrides #300, which should probably be re-opened).

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