Fix typo in generating-clients.md

[lolgab]

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

[codecov-commenter]

Codecov Report

Merging #648 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #648   +/-   ##
=======================================
  Coverage   81.13%   81.13%           
=======================================
  Files          78       78           
  Lines        5163     5163           
  Branches      130      130           
=======================================
  Hits         4189     4189           
  Misses        974      974           

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[blast-hardcheese]

Thank you!