Change card title from Testnet to Mainnet (#291)

Signed-off-by: Ejaz Merchant <[email protected]>

Author: EMerchant90

hedera/getting-started-evm-developers/add-hedera-to-metamask.mdx

@@ -8,7 +8,7 @@ Hedera is fully compatible with web3 wallets like MetaMask. Just add the JSON-RP
 ## Mainnet
 
 <Card
-  title="ADD HEDERA TESTNET"
+  title="ADD HEDERA MAINNET"
   href="https://chainlist.org/chain/295"
   icon="plus-large"
   horizontal