03_numberChecker/README.md : Sentence correction

[spidercam]

Missing 'at' in the following README sentence:

"It is usually easier if you enable only one test a time, then edit your code so that it passes."

Should be: "It is usually easier if you enable only one test at a time, then edit your code so that it passes."

Because

The purpose of this PR is to correct a sentence in the 03_numberChecker exercise README.md file.

This PR

• Changes this sentence in the 2nd paragraph: "It is usually easier if you enable only one test a time, then edit your code so that it passes." --> "It is usually easier if you enable only one test at a time, then edit your code so that it passes."

Issue

Does not close an existing issue.

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. 01_helloWorld: Update test cases
• 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 this PR includes any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder

[mao-sz]

Nice spot