Just rename the scaling file to be found by Sphinx.

There are no references to "scaling.txt" file. Whole kernel tree was
checked using:
$ grep -r "scaling\.txt"

Signed-off-by: Otto Sabart <redacted>
---
 Documentation/networking/{scaling.txt => scaling.rst} | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 rename Documentation/networking/{scaling.txt => scaling.rst} (100%)

diff --git a/Documentation/networking/scaling.txt b/Documentation/networking/scaling.rst
similarity index 100%
rename from Documentation/networking/scaling.txt
rename to Documentation/networking/scaling.rst
-- 
2.17.2