Doc: remove useless role in TopologicalSorter.done() docstring

Change :exec:`ValueError` to ValueError in TopologicalSorter.done() docstring

Author: nikolasavic3

Lib/graphlib.py

@@ -154,7 +154,7 @@ def done(self, *nodes):
         This method unblocks any successor of each node in *nodes* for being returned
         in the future by a call to "get_ready".
 
-        Raises :exec:`ValueError` if any node in *nodes* has already been marked as
+        Raises ValueError if any node in *nodes* has already been marked as
         processed by a previous call to this method, if a node was not added to the
         graph by using "add" or if called without calling "prepare" previously or if
         node has not yet been returned by "get_ready".