Replaced `c_in` with `in_channels`

as proposed in [Better example code? #13](https://huggingface.co/stabilityai/stable-cascade/discussions/13#65ce2b43be38705ceecdd4bc) by

@fischpferd
.

decoder/config.json

@@ -42,7 +42,7 @@
     1280,
     1280
   ],
-  "c_in": 4,
+  "in_channels": 4,
   "c_out": 4,
   "c_pixels": 3,
   "c_r": 64,