Fix broken link about installing mamba

The URL for the mamba installation page changed, and the old URL
does not work as a redirect. This updates it.

This change is analogous to:
EliahKagan/palgoviz#190

Together with dmvassallo#266, this should make the "Check Markdown" workflow
all-green again.

README.md

@@ -105,8 +105,8 @@ conda activate EmbeddingScratchwork
 pip install -e .
 ```
 
-[`mamba`](https://mamba.readthedocs.io/en/latest/installation.html) may be used
-in place of `conda` if it is installed.
+[`mamba`](https://mamba.readthedocs.io/en/latest/installation/mamba-installation.html)
+may be used in place of `conda` if it is installed.
 
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