docs: Update examples using inline attachment with ContentID

[anchepiece]

Fixes #251

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 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:

This is a minor adjustment to attachment usage with SetContentID, hopefully making its use a bit clearer. The "Content-ID" header in a mail message is commonly used to reference the "Content-Disposition: inline;" embedded data within the HTML body. In many implementations the "Content-ID" header is omitted when the disposition is set to "attachment".

References

https://tools.ietf.org/html/rfc2183

[codecov]

Codecov Report

Merging #282 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #282   +/-   ##
=======================================
  Coverage   70.95%   70.95%           
=======================================
  Files           3        3           
  Lines         451      451           
=======================================
  Hits          320      320           
  Misses        127      127           
  Partials        4        4

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[thinkingserious]

Hello @anchepiece,

Thanks again for the PR!

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Team SendGrid DX

[thinkingserious]

Hello @anchepiece,

Thanks again for the PR!

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[childish-sambino]

LGTM