Fix helper mail_example redirection link

[joshuadeguzman]

Fixes #635
Closes #636

Redirection link of the helper's mail_example.py file

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:

• Fixes the broken example url path in the mail helper README.
• Closes PR Fix broken link for mail example #636 which does not redirect the changed url path to the helper's example file.

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #656 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #656   +/-   ##
=======================================
  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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[misterdorm]

Duplicate of #633

[thinkingserious]

Hello @joshuadeguzman,

Thanks again for the PR!

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[joshuadeguzman]

Thank you @thinkingserious!

[thinkingserious]

Hello @joshuadeguzman,

Thanks again for the PR!

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[thinkingserious]

Hello @joshuadeguzman,

Thanks again for the PR!

You have earned 1 out of the 5 points needed to receive glorious SendGrid Hacktoberfest swag.

Please take a moment to checkout this link to find more issues to get you past the required threshold or to simply continue the celebration.

Also, please be sure you have officially registered with us here by November 1, 2018 to qualify.

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