fix error message

noxfile.py

@@ -58,7 +58,7 @@ def check_external_doc_dependencies(session: nox.Session):
     except FileNotFoundError:
         session.error(
             "Unable to build documentation: "
-            "command `dot` from the `graphviz` package (https://www.graphviz.org/) could not be found."
+            "Command `dot` from the `graphviz` package (https://www.graphviz.org/) is not available"
         )