docs/gallium: fixup broken markup

Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19494>

docs/gallium/screen.rst

@@ -690,6 +690,8 @@ If a value greater than 0 is returned, the driver can have multiple
   constant buffers bound to shader stages. The CONST register file is
   accessed with two-dimensional indices, like in the example below.
 
+  ::
+
     DCL CONST[0][0..7]       # declare first 8 vectors of constbuf 0
     DCL CONST[3][0]          # declare first vector of constbuf 3
     MOV OUT[0], CONST[0][3]  # copy vector 3 of constbuf 0