docs: fix simple typo, retrieveing -> retrieving

There is a small typo in svg/symbolic-icons-extract.py.

Should read `retrieving` rather than `retrieveing`.

svg/symbolic-icons-extract.py

@@ -73,7 +73,7 @@ def __init__(self, **kwargs):
 
 
 def get_by_attrib(parent, pred):
-    """Utility function for retrieveing child elements by attribute predicates
+    """Utility function for retrieving child elements by attribute predicates
     """
     return [c for c in parent.getchildren() if pred(c.attrib)]