Rename Unix to Linux

this keeps the tabs under "Default paths" and "Avoiding caching" in sync

to reproduce the issue this commit solves, go to this page:
https://pip.pypa.io/en/stable/topics/caching/
then change the tabs under one of the sections, notice how changing to Unix / Linux breaks sync due to PipCLIDirective using the name [Linux](https://github.com/pypa/pip/blob/95a58e7ba58e769af882502f7436559f61ecca8a/docs/pip_sphinxext.py#L231) for Unix based OS's

Author: iSWORD

docs/html/topics/caching.md

@@ -67,7 +67,7 @@ You can use `pip cache dir` to get the cache directory that pip is currently con
 
 ### Default paths
 
-````{tab} Unix
+````{tab} Linux
 ```
 ~/.cache/pip
 ```