Binary Search spec: Use verifying doubles consistently where introduced

[macauleydev]

Because

• This file, 15b_binary_search_spec.rb, as explained in its comments, "uses verifying doubles to help you get acqauintance with the concept". But it replaces only 2 of the file's 4 doubles with verifying doubles.
• The inconsistency is not explained, so it creates unnecessary confusion.
• For what it's worth, a related file, 15a_binary_game_spec, uses verifying doubles consistently.

This PR

• Removes the unexplained inconsistency from 15b by replacing the remaining 2 doubles with verifying doubles.
• (Also removes some trailing spaces. Not sure if Prettier or RuboCop deserves credit/blame. Hopefully not a problem.)

Issue

N/A

Additional Information

I found a Discord discussion where a student mentioned confusion about this inconsistency and @rlmoser99 invited a PR, but it looks like none was made.

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. String spec: Update instructions for clarity
• 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 changes in the spec folder, they are also updated in the corresponding file in the spec_answers folder (with passing tests). [N/A, as this file is informational and has no corresponding answer file]

[rlmoser99]

Thanks. I am not sure how those got out of sync, so thanks for making a PR.