Fix link in makefile (#620)

Author: j-bowhay

doc/Makefile

@@ -11,7 +11,7 @@ themes/scientific-python-hugo-theme:
 	  echo "It looks as though you are missing the themes directory."; \
 	  echo "You need to add the scientific-python-hugo-theme as a submodule."; \
 	  echo ; \
-	  echo "Please see https://theme.scientific-python.org/getstarted/"; \
+	  echo "Please see https://theme.scientific-python.org/user_guide/getstarted/"; \
 	  echo ; \
 	  exit 1; \
 	fi