doc: update branch name from master to main in CONTRIBUTING guide

[BrianKopp]

Issue #, if available:

Description of changes:

While reviewing the CONTRIBUTING.md file for contribution, I noticed it incorrectly says to fork the master branch, when what it probably means is main branch.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rushmash91]

Thanks @BrianKopp !

/lgtm

[rushmash91]

@a-hilaly can we merge this..

[a-hilaly]

/lgtm

[ack-prow[bot]]

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This pull-request has been approved by: a-hilaly, BrianKopp, rushmash91

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