Updated check block in example for Istanbul

[todofixthis]

> const EthereumBlock = require('ethereumjs-block')

> let headers = new EthereumBlock.Header('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')

> headers.hash().toString('hex')
'451226b98bf4f784314e9ca2daaa30dc664a387c342ef775ba2d88682a27c084'

https://etherscan.io/block/0x451226b98bf4f784314e9ca2daaa30dc664a387c342ef775ba2d88682a27c084 👍🏻

[coveralls]

Pull Request Test Coverage Report for Build 169

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.687%

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Totals
Change from base Build 158:	0.0%
Covered Lines:	907
Relevant Lines:	961

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💛 - Coveralls

[holgerd77]

Sorry that this never made it in. Outdated, will close.