[raspbian] Updated experimental warning

Author: Ritwick DSouza

docs/source/backends/raspbian.rst

@@ -8,7 +8,7 @@ The ``Raspbian`` backend allows to Raspbian compatible configuration files.
 
 .. warning::
     This backend is in experimental stage: it may have bugs and it will
-    receive backward incompatible updates in during the first 6 months
+    receive backward incompatible updates during the first 6 months
     of development (starting from September 2017).
     Early feedback and contributions are very welcome and will help
     to stabilize the backend faster.