Granular marking selector validation is incorrect

[chisholm]

Selector validation fails to validate compliant selectors. Here is an example taken from STIX 2.1 spec section 7.2.3.1:

import stix2

vuln = stix2.parse(
    {
      "id": "vulnerability--ee916c28-c7a4-4d0d-ad56-a8d357f89fef",
      "spec_version": "2.1",
      "created": "2016-02-14T00:00:00.000Z",
      "modified": "2016-02-14T00:00:00.000Z",
      "type": "vulnerability",
      "name": "CVE-2014-0160",
      "description": "The (1) TLS...",
      "external_references": [{
        "source_name": "cve",
        "external_id": "CVE-2014-0160"
      }],
      "labels": ["heartbleed", "has-logo"]
    }
)

marked_vuln = vuln.add_markings(stix2.TLP_GREEN, "external_references.[0].source_name")

This results in error:

Traceback (most recent call last):
  File "...\testx_selector_bug.py", line 42, in <module>
    marked_vuln = vuln.add_markings(stix2.TLP_GREEN, "external_references.[0].source_name")
  File "...\cti-python-stix2\stix2\markings\__init__.py", line 153, in add_markings
    return granular_markings.add_markings(obj, marking, selectors)
  File "...\cti-python-stix2\stix2\markings\granular_markings.py", line 165, in add_markings
    utils.validate(obj, selectors)
  File "...\cti-python-stix2\stix2\markings\utils.py", line 50, in validate
    raise exceptions.InvalidSelectorError(obj, s)
stix2.exceptions.InvalidSelectorError: Selector external_references.[0].source_name in Vulnerability is not valid!