Fix a typo in the global README.
Fix a typo in the global README.

Summary: s's should be s'.

Reviewers: #material_components_ios_owners, ajsecord

Reviewed By: #material_components_ios_owners, ajsecord

Projects: #material_components_ios_owners

Differential Revision: http://codereview.cc/D23

material-components/material-components-ios,randallli/material-components-ios,chriscox/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,sigito/material-components-ios,chriscox/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,brianjmoore/material-components-ios,Eccelor/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,sigito/material-components-ios,sigito/material-components-ios,chriscox/material-components-ios,Eccelor/material-components-ios,chriscox/material-components-ios,samnm/material-components-ios,material-components/material-components-ios,chriscox/material-components-ios,samnm/material-components-ios,sigito/material-components-ios,brianjmoore/material-components-ios,material-components/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,Eccelor/material-components-ios