Calculator: Improve expressions and sentences

[softy-dev]

Because

Some nitpick changes to improve clarity, decrease redundancy, and make the instructions more readable.

This PR

• Removes redundancy that doesn't add to meaning;
• Makes some sentences more concise;
• Improves consistency (for instance, instead of using both "Equals" and "=", only the latter is used).

Issue

Closes #XXXXX

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
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[MaoShizhong]

I like these - nice improvements!