Improve assert_shape error message when Ellipsis is used as shape matcher

Using `chex.assert_shape(x, ...)` raises:

AssertionError: expected shapes should be a list or tuple of ints, got Ellipsis

However, Ellipsis works when wrapped in a tuple:

chex.assert_shape(x, (...,))

This makes it unclear whether `...` is unsupported or just incorrectly formatted.

Suggested improvement:
Clarify the error message to indicate a shape specification must be a sequence and suggest using `(...,)`.

Example improved message:
Expected a shape specification tuple, e.g. (...,) instead of bare Ellipsis (...)

Why this helps:
Users commonly interpret `...` as a valid wildcard shape and the current message implies a type mistake rather than guiding correct usage.