Update old URL in docstrings for API doc

Author: Tieqiong

src/diffpy/structure/parsers/p_cif.py

@@ -15,7 +15,7 @@
 
 """Parser for basic CIF file format
 
-http://www.iucr.org/iucr-top/cif/home.html
+https://www.iucr.org/resources/cif
 """
 
 import re

src/diffpy/structure/parsers/p_pdb.py

@@ -17,8 +17,8 @@
 
 References
 
-    http://www.wwpdb.org/documentation/format23/v2.3.html
-    http://www.wwpdb.org/documentation/format30/index.html
+    https://www.wwpdb.org/documentation/file-format-content/format23/v2.3.html
+    https://www.wwpdb.org/documentation/file-format-content/format30/index.html
 """
 
 import sys