fix: psycopg 3 checkout dir after changing repo url

Author: dvarrazzo

Makefile

@@ -57,7 +57,7 @@ psycopg3/docs/.venv: psycopg3/README.rst
 
 psycopg3/README.rst:
 	test -d psycopg3/.git \
-		|| git clone -b $(DOC3_BRANCH) https://github.com/psycopg/psycopg.git
+		|| git clone -b $(DOC3_BRANCH) https://github.com/psycopg/psycopg.git psycopg3
 	git -C psycopg3 checkout $(DOC3_BRANCH)
 	git -C psycopg3 pull