Update README.md with up-to-date URLs

Author: rhaas80

README.md

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 # [SpacetimeX](https://github.com/EinsteinToolkit/SpacetimeX)
 
-**SpacetimeX** is a collection of thorns which prepare initial data for, evolve and analyze spacetime metric. They are built upon [CarpetX](https://github.com/eschnett/CarpetX). 
-For the matter evolution thorns, please checkout [AsterX](https://github.com/jaykalinani/AsterX).
+**SpacetimeX** is a collection of thorns which prepare initial data for, evolve and analyze spacetime metric. They are built upon [CarpetX](https://github.com/EinsteinToolkit/CarpetX). 
+For the matter evolution thorns, please checkout [AsterX](https://github.com/EinsteinToolkit/AsterX).
 
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