Change contentId to content_id

[taktran]

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:

Updated docs to match the API

[SendGridDX]

All committers have signed the CLA.

[taktran]

Haven't done everything on the checklist yet, but it's a very simple change. Can do it if need be.

contentId didn't work for @sendgrid/mail v6.1.4, but content_id did as per the spec https://sendgrid.com/docs/API_Reference/api_v3.html

Maybe you want to change the nodejs API though.

[thinkingserious]

Thanks @taktran!

This is now on our backlog for review.

[massm90s]

It seems legit

[thinkingserious]

Thanks for taking the time to review @massm90s.

[clee]

LGTM

[thinkingserious]

Hello @taktran,

Thanks again for the PR!

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