Updating language for chain neutrality (#263)

Fix

Author: MichaelMacaulay

pages/en/cookbook/migrating-a-subgraph.mdx

@@ -11,7 +11,7 @@ The process of migration is quick and your subgraphs will forever benefit from t
 ### Assumptions
 
 - You have already deployed a subgraph on the hosted service.
-- The subgraph is indexing Ethereum mainnet.
+- The subgraph is indexing a chain available (or available in beta) on The Graph Network.
 - The subgraph does not have IPFS or full-text search dependencies (these are not fully supported on the decentralized network yet).
 
 ## Migrating an Existing Subgraph to The Graph Network