Fix pdf-doc build by skiping the sample file

Tempest pdf doc build includes the sample config
file which is large in size and end up with
error 'Dimension too large'

- https://zuul.opendev.org/t/openstack/build/84d73ec987d04c05acc54b231d33067e/log/sphinx-build-pdf.log#1874

Its happening while adding new config option in
https://review.opendev.org/#/c/686073/ which means we
are at the edge of pdf build limit.

We should not include the sample config file as part
of pdf build.

Change-Id: I59383505989705d35d86b19b5c1aaafebd2bd307
Closes-Bug: #1886212
diff --git a/doc/source/sampleconf.rst b/doc/source/sampleconf.rst
index c290140..45164a3 100644
--- a/doc/source/sampleconf.rst
+++ b/doc/source/sampleconf.rst
@@ -10,4 +10,6 @@
 
 The sample configuration can also be viewed in `file form <_static/tempest.conf.sample>`_.
 
-.. literalinclude:: _static/tempest.conf.sample
+.. only:: html
+
+   .. literalinclude:: _static/tempest.conf.sample