docs: Fix incorrect file extension Update README.md (#13685)

Fix incorrect file extension in rollup-node spec link

Author: santamasa

op-node/README.md

@@ -17,7 +17,7 @@ It functions as a Consensus Layer client of an OP Stack chain.
 This builds, relays and verifies the canonical chain of blocks.
 The blocks are processed by an execution layer client, like [op-geth].
 
-[rollup-node spec]: https://github.com/ethereum-optimism/specs/blob/main/specs/protocol/rollup-node.m
+[rollup-node spec]: https://github.com/ethereum-optimism/specs/blob/main/specs/protocol/rollup-node.md
 [op-geth]: https://github.com/ethereum-optimism/op-geth
 
 ## Quickstart