chore(reth): fix flashblock test RPC call

[mur-me]

During my flashblocks setup I've noticed this small issue, this extra header for curl fixes it

[cb-heimdall]

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[mslord213]

Looks good — adding the Content-Type: application/json header makes the example conformant with JSON-RPC and avoids potential 415 errors from stricter servers.

One minor thought: if there are other curl JSON-RPC examples in the docs, it might be worth checking they also include the header so that users get consistent guidance. Otherwise this change looks solid.