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-161783079
Comment:
@GuillaumeGomez I'm sorry I wasn't clear, this isn't your issue.  My comment was to @bjacke who is on the Samba Team, and so can get the code reviewed and submitted via the normal methods.

We realise this is very confusing (and contrary to standard github workflow), but we will get your change in shortly. 

Thanks!