Update substreams-powered-subgraphs.mdx

-adding link to supported networks

Author: idalithb

website/pages/en/subgraphs/cookbook/substreams-powered-subgraphs.mdx

@@ -158,7 +158,7 @@ Substreams-powered subgraphs introduce a new `kind` of data source, "substreams"
 
 This data source must specify the indexed network, the Substreams package (`spkg`) as a relative file location, and the module within that Substreams package which produces subgraph-compatible entity changes (in this case `map_entity_changes`, from the Substreams package above). The mapping is specified, but simply identifies the mapping kind ("substreams/graph-entities") and the apiVersion.
 
-> Currently, Subgraph Studio and The Graph Network support Substreams-powered subgraphs which index `mainnet` (Mainnet Ethereum).
+> Currently, Subgraph Studio and The Graph Network support Substreams-powered subgraphs. Review [supported networks](/en/supported-networks/) for additional details. 
 
 ```yaml
 specVersion: 0.0.4