Fix broken link for mail example

[mattjegan]

Fixes

Fixes #635

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.
• N/A I have added tests that prove my fix is effective or that my feature works
• N/A I have added necessary documentation about the functionality in the appropriate .md file
• N/A I have added in line documentation to the code I modified

Short description of what this PR does:

• Fixes the broken 'examples' link in the mail helper readme.

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #636 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #636   +/-   ##
=======================================
  Coverage   84.77%   84.77%           
=======================================
  Files          35       35           
  Lines        1156     1156           
  Branches      172      172           
=======================================
  Hits          980      980           
  Misses         90       90           
  Partials       86       86

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[thinkingserious]

Hello @mattjegan,

Thanks again for the PR!

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Team SendGrid DX