Clarify theta argument validation error message

[raphaelreinauer]

In src/ect/ect.py:84-87:

        if theta is not None and graph_dim != 2:
            raise ValueError(
                "Theta must be provided for 2D graphs. "
                "Use 'directions' or 'num_dirs' to specify directions."
            )

The condition rejects theta for non-2D graphs, but the message says theta "must be provided for 2D graphs", which is the opposite.

Suggested fix:

raise ValueError(
    "theta is only supported for 2D graphs. "
    "Use 'directions' or 'num_dirs' for higher dimensions."
)

Part of this JOSS review.