Minor issue with links in approach.rst

[handwerkerd]

Summary

I noticed a couple of links with no target in https://github.com/tsalo/tedana/blob/modularize-metrics/docs/approach.rst#id4
Around line 188 is:

combineddata usingPrincipal Components Analysis (PCA)_ and then an Independent Components Analysis (ICA)`_.

Neither the PCA or ICA links have targets. I'm not sure if the plan was to link to general PCA & ICA guides or internally into to another point in the documentation.

Next Steps

In the short-term, it probably makes sense to either remove the links or link to a general guide.