Typo in documentation

[yfarjoun]

Description

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Please explain the changes you made here.
Explain the motivation for making this change. What existing problem does the pull request solve?
Mention any issues fixed, addressed or otherwise related to this pull request, including issue numbers or hard links for issues in other repos.
You can delete these instructions once you have written your PR description.

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Checklist (never delete this)

Never delete this, it is our record that procedure was followed. If you find that for whatever reason one of the checklist points doesn't apply to your PR, you can leave it unchecked but please add an explanation below.

Content

• Added or modified tests to cover changes and any new functionality
• Edited the README / documentation (if applicable)
• All tests passing on Travis

Review

• Final thumbs-up from reviewer
• Rebase, squash and reword as applicable

For more detailed guidelines, see https://github.com/broadinstitute/picard/wiki/Guidelines-for-pull-requests