docs: ntb.txt: use Sphinx notation for the two ascii figures

In order to make it to build with Sphinx, we need to fix the
notation for two ascii artwork.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/ntb.txt

@@ -41,9 +41,10 @@ mainly used to perform the proper memory window initialization. Typically
 there are two types of memory window interfaces supported by the NTB API:
 inbound translation configured on the local ntb port and outbound translation
 configured by the peer, on the peer ntb port. The first type is
-depicted on the next figure
+depicted on the next figure::
+
+ Inbound translation:
 
-Inbound translation:
  Memory:              Local NTB Port:      Peer NTB Port:      Peer MMIO:
   ____________
  | dma-mapped |-ntb_mw_set_trans(addr)  |
@@ -58,9 +59,10 @@ maps corresponding outbound memory window so to have access to the shared
 memory region.
 
 The second type of interface, that implies the shared windows being
-initialized by a peer device, is depicted on the figure:
+initialized by a peer device, is depicted on the figure::
+
+ Outbound translation:
 
-Outbound translation:
  Memory:        Local NTB Port:    Peer NTB Port:      Peer MMIO:
   ____________                      ______________
  | dma-mapped |                |   | MW base addr |<== memory-mapped IO