Fix the "contacts:MSExchageCertificate" typo

[mattguo]

Should be "contacts:MSExchangeCertificate".

Otherwise Exchange will reject all SOAP request containing this FieldURI

[azurecla]

Hi @mattguo, I'm your friendly neighborhood Azure Pull Request Bot (You can call me AZPRBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no contribution license agreement is required at this point. Real humans will now evaluate your PR.

TTYL, AZPRBOT;

[codecov-io]

Current coverage is 10.42%

Merging #507 into master will not affect coverage as of bed8104

@@            master    #507   diff @@
======================================
  Files          550     550       
  Stmts        20432   20432       
  Branches      2625    2625       
  Methods          0       0       
======================================
  Hit           2131    2131       
  Partial        153     153       
  Missed       18148   18148       

Review entire Coverage Diff as of bed8104

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[serious6]

@mattguo thanks for your contribution