use 'sphinx-build' rather than invoking 'sphinx.main'

Sphinx 2 got rid of 'sphinx.main' so we can no longer use it.

doc/Makefile

@@ -10,7 +10,7 @@
 
 # You can set these variables from the command line.
 SPHINXOPTS    =
-SPHINXBUILD   = python3 -c 'import sys, sphinx; sys.exit(sphinx.main(sys.argv))'
+SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = build