docs: fix grammar in Readme

[jmauerhan]

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.

Short description of what this PR does:

• Grammar fix

[codecov]

Codecov Report

Merging #122 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #122   +/-   ##
======================================
  Coverage    84.7%   84.7%           
======================================
  Files           3       3           
  Lines         170     170           
======================================
  Hits          144     144           
  Misses         26      26

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[thinkingserious]

Hello @jmauerhan,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX