doc: Update the name of BSD ports

Both FreeBSD and OpenBSD ports have new names:
- https://www.freshports.org/devel/py-yamllint
- https://openports.pl/path/devel/py-yamllint

Author: DimitriPapadopoulos

docs/quickstart.rst

@@ -27,13 +27,13 @@ On FreeBSD:
 
 .. code:: sh
 
-  pkg install py36-yamllint
+  pkg install py-yamllint
 
 On OpenBSD:
 
 .. code:: sh
 
-  doas pkg_add py3-yamllint
+  doas pkg_add py-yamllint
 
 Alternatively using pip, the Python package manager: