docs: fix stacindex link (#484)

### Motivation

Stac Index badge does not currently link to STAC index 

### Modifications

adjust bad to link to stac index.

### Verification

<!-- TODO: Say how you tested your changes. -->

README.md

@@ -1,6 +1,6 @@
 # New Zealand Imagery
 
-![Static Badge](https://img.shields.io/badge/New_Zealand_Imagery-%2309B3AD?style=flat&label=Open%20in%20STAC%20Index&labelColor=%23144E63&link=https%3A%2F%2Fstacindex.org%2Fcatalogs%2Fnz-imagery%23%2F)
+[![STAC Index Badge](https://img.shields.io/badge/New_Zealand_Imagery-%2309B3AD?style=flat&label=Open%20in%20STAC%20Index&labelColor=%23144E63&link=https%3A%2F%2Fstacindex.org%2Fcatalogs%2Fnz-imagery%23%2F)](https://stacindex.org/catalogs/nz-imagery#/)
 
 Toitū Te Whenua makes New Zealand's most up-to-date publicly owned aerial imagery freely available to use under an open licence. You can access this through the [LINZ Data Service](https://data.linz.govt.nz/data/category/aerial-photos/?s=n), [LINZ Basemaps](https://basemaps.linz.govt.nz/#@-41.8899962,174.0492437,z5) or the [Registry of Open Data on AWS](https://registry.opendata.aws/nz-imagery/).