Update README.md

Refer to ethereum-optimism/ecosystem-contributions#80

README.md

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-Optimism is a Layer 2 platform for Ethereum.
+OP Mainnet is a Layer 2 platform for Ethereum.
 
-Optimism is, in a nutshell, an application inside of Ethereum that executes transactions more efficiently than Ethereum itself. It's based on the concept of the [Optimistic Rollup](https://research.paradigm.xyz/rollups), a construction that allows us to "optimistically" publish transaction results without actually executing those transactions on Ethereum (most of the time). Optimism makes transactions cheaper, faster, and smarter.
+OP Mainnet is, in a nutshell, an application inside of Ethereum that executes transactions more efficiently than Ethereum itself. It's based on the concept of the [Optimistic Rollup](https://research.paradigm.xyz/rollups), a construction that allows us to "optimistically" publish transaction results without actually executing those transactions on Ethereum (most of the time). OP Mainnet makes transactions cheaper, faster, and smarter.
 
 Please note that this repository is undergoing rapid development.