Fix unsubscribe preferences link in example

[dylans]

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:

• Example for unsubscribe preferences did not actually include the correct replacement link. This PR updates to the value shown in https://docs.sendgrid.com/ui/sending-email/recipient-subscription-preferences#the-email-preferences-link