docs(base-contracts): OptimismMintableERC20Factory mainnet address inconsistency and incorrect footnote

## Problem

File: docs/base-chain/network-information/base-contracts.mdx

The L2 contract address table lists different addresses for `OptimismMintableERC20Factory` on mainnet vs testnet:

- Mainnet: `0xF10122D428B4bc8A9d050D06a2037259b4c4B83B`
- Testnet: `0x4200000000000000000000000000000000000012`

According to the OP Stack specification, the canonical predeploy address for `OptimismMintableERC20Factory` is `0x4200000000000000000000000000000000000012` on all OP Stack chains (both mainnet and testnet).

## Additional issue

The footnote at the bottom of the L2 table states:
> *L2 contract addresses are the same on both mainnet and testnet.*

This is contradicted by the table itself, since the two `OptimismMintableERC20Factory` addresses differ.

## Expected behavior

Either:
1. Update the mainnet `OptimismMintableERC20Factory` address to `0x4200000000000000000000000000000000000012` if the current entry is outdated, OR
2. Add a clarifying note explaining that the mainnet entry is a legacy/wrapper contract, and update the footnote to reflect the exception.

## References

- OP Stack predeploys: https://specs.optimism.io/protocol/predeploys.html