docs: Note build requirement for poetry<2

README.rst

@@ -65,7 +65,7 @@ If your system uses a different kind of CPU, you should be able to install from
     make
 
     # Build the httpstan wheel on your system
-    python3 -m pip install poetry
+    python -m pip install "poetry~=1.7.1"
     python3 -m poetry build
 
     # Install the wheel