Fix little typo in README file

[github at samba.org]

New comment by abartlet on Samba Github repository

https://github.com/samba-team/samba/pull/41#issuecomment-161739851
Comment:
Sadly one of the downsides of the way that the Samba Team uses github is that you can't directly merge.  You need to go via autobuild.

This merge will be undone as soon as I disable to force-push protection and the mirror script runs again.