Fix grid size in README (#321)

`imshow_gui` specifies that `gridsize = (nx, ny)`, meaning
(columns, rows). One of the uses in the README had it reversed.

Author: timholy

README.md

@@ -186,7 +186,7 @@ change in the other.
 Alternatively, you can place both displays in a single window:
 ```julia
 zr, slicedata = roi(mri, (1,2))
-gd = imshow_gui((200, 200), (1,2); slicedata=slicedata)
+gd = imshow_gui((200, 200), (2, 1); slicedata=slicedata)
 imshow(gd["frame"][1,1], gd["canvas"][1,1], mri, nothing, zr, slicedata)
 imshow(gd["frame"][1,2], gd["canvas"][1,2], mriseg, nothing, zr, slicedata)
 ```