Fix env variables link on TROUBLESHOOTING.md

[imagentleman]

Fixes

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Fixes the "environment variables" link on TROUBLESHOOTING.md, which points to a section that does not exist on the README.md. Changed it to point to the equivalent section on CONTRIBUTING.md.

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[reedsa]

Duplicates work done in #789

[imagentleman]

Closing since it's a duplicate.