CONTRIBUTING: Spell "offered" correctly

Reported-by: Radovan Bast

CONTRIBUTING.rst

@@ -28,7 +28,7 @@ License
 
 We do not require any formal copyright assignment or contributor license
 agreement.  Any contributions intentionally sent upstream are presumed
-to be offerred under terms of the OSI-approved BSD 3-clause License.
+to be offered under terms of the OSI-approved BSD 3-clause License.
 See `Copyright.txt`_ for details.
 
 .. _`Copyright.txt`: Copyright.txt