Update 05.1_intro.md

add missing link ref

intermediate/data_cleaning/05.1_intro.md

@@ -64,7 +64,7 @@ These are guidelines to keep in mind while you are organizing your data. For det
 
 **6. Make us of & work within the framework of other tools**
 
-- Specification systems such as [CF]() and [STAC](https://stacspec.org/en), and related tools such as [Open Data Cube](https://www.opendatacube.org/), [PySTAC](https://pystac.readthedocs.io/en/stable/), [cf_xarray](https://cf-xarray.readthedocs.io/en/latest/),[stackstac](https://stackstac.readthedocs.io/en/latest/) and more make tidying possible and smoother, especially with large, cloud-optimized datasets.
+- Specification systems such as [CF](https://cfconventions.org/) and [STAC](https://stacspec.org/en), and related tools such as [Open Data Cube](https://www.opendatacube.org/), [PySTAC](https://pystac.readthedocs.io/en/stable/), [cf_xarray](https://cf-xarray.readthedocs.io/en/latest/),[stackstac](https://stackstac.readthedocs.io/en/latest/) and more make tidying possible and smoother, especially with large, cloud-optimized datasets.
 -
 
 ## Other guidelines and rules of thumb