Update link in definition of text alternatives

[JAWS-test2]

Fixes #1029

[patrickhlauke]

@kfranqueiro is there a better, more evergreen way to link from the term to that particular understanding part? I'm always a bit loathe to see fully qualified URLs to a specific version of the spec in these things

[kfranqueiro]

This PR was sent over 4 years ago, and since then, this exact update has already been made, so I think this should be closed.

To @patrickhlauke's question, AFAIK this isn't something that is explicitly supported; most importantly, this would also need to work within the guidelines themselves, and I'm not sure whether that's realistic.

One thing that would probably be worth pursuing if at all possible would be rewriting explicit 2.1 links to the appropriate version within the context of the informative documentation.

[kfranqueiro]

Already included in #1830