Update PURL-SPECIFICATION.rst

Author: jkowalleck

PURL-SPECIFICATION.rst

@@ -423,6 +423,9 @@ all package types:
 - ``vers`` allows the specification of a version range.
   The value MUST adhere to the `Version Range Specification <VERSION-RANGE-SPEC.rst>`_.
   This qualifier is mutually exclusive with the ``version`` component.
+  For example::
+
+       pkg:pypi/django?vers=vers%3Apypi%2F%3E%3D1.11.0%7C%21%3D1.11.1%7C%3C2.0.0
 
 - ``repository_url`` is an extra URL for an alternative, non-default package
   repository or registry. When a package does not come from the default public