Add necesarry --force to tap instructions

[leoheitmannruiz]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Executing brew tap homebrew/core tells you to execute brew tap --force homebrew/core (in case you are interested in contributing, which you are if you're reading the Formula Cookbook).

[carlocab]

Thanks!