Ruby on Rails Installing Postgresql: Fix link to rails source code on GH

[dwlknsn]

Because

Minor tweak to an external link to the rails source code. The source code has changed, and the line reference was incorrect.

This PR

• Updates the external link to the rails source code on GH to point to the correct location where Rails.env is determined.

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
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• If any lesson files are included in this PR, they follow the Layout Style Guide

[KevinMulhern]

Thanks for contributing @dwlknsn!