Documentation: Fix make rule for sphinx-autobuild

Execute sphinx-autobuild for livesphinx make rule

Change-Id: I725392f1f132101eede8fed75e8d225c972ad1fe
Signed-off-by: Tom Hiller <thrilleratplay@gmail.com>
Reviewed-on: https://review.coreboot.org/28326
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
diff --git a/Documentation/Makefile.sphinx b/Documentation/Makefile.sphinx
index 8c10a9f..5236f3e 100644
--- a/Documentation/Makefile.sphinx
+++ b/Documentation/Makefile.sphinx
@@ -57,7 +57,7 @@
 
 .PHONY: livehtml
 livehtml:
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)
+	$(SPHINXAUTOBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)
 	@echo
 	@echo "Autobuild finished. The HTML pages are in $(BUILDDIR)."