Fix article ID parsing issue

This fix avoids returning also the IDs of cited
papers
(they are within the ReferenceList element of the xml).

Fixes #22

An alternative XPath to be used:
path = ".//PubmedData/ArticleIdList/ArticleId[@idtype='pubmed']"

Author: iacopy

pymed/article.py

@@ -47,7 +47,7 @@ def __init__(
                 self.__setattr__(field, kwargs.get(field, None))
 
     def _extractPubMedId(self: object, xml_element: TypeVar("Element")) -> str:
-        path = ".//ArticleId[@IdType='pubmed']"
+        path = "MedlineCitation/PMID"
         return getContent(element=xml_element, path=path)
 
     def _extractTitle(self: object, xml_element: TypeVar("Element")) -> str: