Change LEL reference

[gervandiepen]

The original description of the LatticeExpression Language is in casacore note 223: https://casacore.github.io/casacore-notes/223.html. Using this reference better reflects where LEL originates (it is part of casacore, not CASA).
Furthermore, when changes are made to LEL they will be reflected in this note and probably not in the CASA document.
So you might consider changing the LEL reference in the CARTA documentation.

[kswang1029]

@gervandiepen thanks for pointing out this issue. I have updated the reference URL. Please check https://carta.readthedocs.io/en/3.0/image_visualization.html#loading-images-with-the-lattice-expression-language-lel
https://carta.readthedocs.io/en/latest/image_visualization.html#loading-images-with-the-lattice-expression-language-lel

The upcoming v4.0 user manual will also have this fix.