docs: add eth_getTransactionReceipt indexing error code

[roethke]

What changed? Why?
eth_getTransactionReceipt error handling table has a missing error code; the -32000 error indicates that the node is still indexing transactions, which can occur when querying for receipts during node startup or syncing.

{"jsonrpc":"2.0","id":13,"error":{"code":-32000,"message":"transaction indexing is in progress","data":"transaction indexing is in progress"}}

Notes to reviewers

How has it been tested?
Locally

[cb-heimdall]

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