Fix modification of dict during iteration in graph extension

(refs idaholab#16410)

python/MooseDocs/extensions/graph.py

@@ -239,7 +239,7 @@ def __init__(self, name):
 
     def __call__(self, **kwargs):
         """Replace the markers with the supplied key, value pairs."""
-        for key in kwargs:
+        for key in list(kwargs.keys()):
             if key.endswith('_'):
                 kwargs[key[:-1]] = kwargs.pop(key)