docs: update mms example to use media_url

[cnorm35]

Fixes

In examples/example.rb example for 'send an mms' uses media_urls which returns ArgumentError: unknown keyword: :media_urls. This change updates to use media_url which the documentation lists as the current syntax.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

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