project_calculator: Add new bug to watch out for in "Gotchas" section

[KHenkel95]

Because

Added another common bug for users to watch out for when working on this project.

This PR

Added this item under the "Gotchas" section:

• Make sure that your calculator only runs an operation when supplied with two numbers and an operator by the user. Example: you enter a number (2), followed by an operator button (+). You press the operator button (+) a second consecutive time. Your calculator should not evaluate this as (2 + 2) and should not display the result (4). If consecutive operator buttons are pressed, your calculator should not run any evaluations, it should only take the last operator entered to be used for the next operation.

Issue

Closes #29174

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
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• If any lesson files are included in this PR, they follow the Layout Style Guide

[wise-king-sullyman]

Looks great, thanks!