Small fix in images and containers docs

Signed-off-by: Tomasz Kopczynski <tomek@kopczynski.net.pl>

docs/userguide/storagedriver/imagesandcontainers.md

@@ -154,7 +154,7 @@ stored in this writable layer. When the container is deleted the writable layer
  is also deleted. The underlying image remains unchanged.
 
 Because each container has its own thin writable container layer, and all 
-changes are stored this container layer, this means that multiple containers 
+changes are stored in this container layer, this means that multiple containers 
 can share access to the same underlying image and yet have their own data 
 state. The diagram below shows multiple containers sharing the same Ubuntu 
 15.04 image.