Documentation: application-developer: Use correct type to store size

The return type of std::vector::size() is size_t. Use the same type,
instead of unsigned int, to store its return value when retrieving the
number of allocated buffers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Documentation/guides/application-developer.rst

@@ -278,7 +278,7 @@ as the parameter of the ``FrameBufferAllocator::buffers()`` function.
            return -ENOMEM;
        }
 
-       unsigned int allocated = allocator->buffers(cfg.stream()).size();
+       size_t allocated = allocator->buffers(cfg.stream()).size();
        std::cout << "Allocated " << allocated << " buffers for stream" << std::endl;
    }