Ruby on Rails External APIs Lesson: Change link text

[AhmedTheGreatest]

Because

There was a mention to "Chrome's Rest Client" that linked to the Postman website. Postman's chrome externsion is now deprecated and instead of it we should use the Postman app.

This PR

• Changed "a browser tool like Chrome's Rest Client" to "an application like Postman"

Issue

Related to #28050

Additional Information

I directly made a Pull Request as it is a simple and small change.

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project curriculum contributing guide
• [x] 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
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] 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