Fix separator in testing Makefile

Author: awvwgk

src/tests/Makefile.manual

@@ -9,4 +9,4 @@ all test clean:
 	$(MAKE) -f Makefile.manual --directory=quadrature $@
 	$(MAKE) -f Makefile.manual --directory=stats $@
 	$(MAKE) -f Makefile.manual --directory=string $@
-  $(MAKE) -f Makefile.manual --directory=math $@
+	$(MAKE) -f Makefile.manual --directory=math $@