Drop unnecessary 'x' bit from doc config file

This file is not actually executable.

Trivial-Fix

Change-Id: Ic89bfe6523fddf4a11e4e287db96775e3f13cad1
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
diff --git a/doc/source/conf.py b/doc/source/conf.py
old mode 100755
new mode 100644