stack/erp5: Update comment, we use py3

We currently use python 3.8:

[1] https://lab.nexedi.com/nexedi/slapos/blob/d2404ed9/stack/erp5/buildout-py3.cfg#L36
[2] https://lab.nexedi.com/nexedi/slapos/blob/7d78a928c09b63508c69f84aa499a9870b3ae37e/component/python3/buildout.cfg#L21

stack/erp5/buildout-py3.cfg

@@ -165,7 +165,7 @@ zc.recipe.egg = 2.0.3+slapos003
 pyasn1-modules = 0.2.8
 pyasn1 = 0.4.8
 
-## Version up to support Python 3.7
+## Version up to support Python 3.8
 numpy = 1.16.6
 pandas = 0.24.2
 google-api-python-client = 2.41.0