docs: staging: don't use literalinclude

Such directive cause troubles with PDF output.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9a9043face7ace90e3616ba840a8e09cfc8f143f.1592203650.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/staging/index.rst

@@ -19,14 +19,17 @@ Unsorted Documentation
 Atomic Types
 ============
 
-.. literalinclude:: ../atomic_t.txt
+.. include:: ../atomic_t.txt
+   :literal:
 
 Atomic bitops
 =============
 
-.. literalinclude:: ../atomic_bitops.txt
+.. include:: ../atomic_bitops.txt
+   :literal:
 
 Memory Barriers
 ===============
 
-.. literalinclude:: ../memory-barriers.txt
+.. include:: ../memory-barriers.txt
+   :literal: