Fix html-watch target.

documentation/usage-guide/Makefile

@@ -16,7 +16,7 @@ help:
 .PHONY: help Makefile
 
 html-watch:
-	@$(SPHINXAUTOBUILD) "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(SPHINXAUTOBUILD) "$(SOURCEDIR)" "$(BUILDDIR)/html" $(SPHINXOPTS) $(O)
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).