sphinx: fix sys.path so it finds the borg package

docs/conf.py

@@ -15,7 +15,7 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 import sys, os
-sys.path.insert(0, os.path.abspath('..'))
+sys.path.insert(0, os.path.abspath('../src'))
 
 from borg import __version__ as sw_version