Change "mainnet" in template to "mainnet-fork" (#2268)

Author: jribbink

internal/super/generator/templates/test_incrementfi_swap_on_fork.cdc.tmpl

@@ -8,7 +8,7 @@
 //
 // Learn more about fork testing in the README.md file or at:
 // https://developers.flow.com/blockchain-development-tutorials/cadence/fork-testing
-#test_fork(network: "mainnet", height: nil)
+#test_fork(network: "mainnet-fork", height: nil)
 
 import Test
 import "DeFiActions"