[documentation] Clarify privateKeyToAccount private string is hex

[tcrowe]

What does it do?

#1267 brought up a good point that web3.eth.accounts.privateKeyToAccount privateKey does not specify that it's a hex string. This PR fixes that in the documentation.

Does it work?

[coveralls]

Coverage remained the same at 83.071% when pulling 5b5b296 on tcrowe:docs-privatekeytoaccount-hex-1267 into 1d9f6c0 on ethereum:1.0.

[joshstevens19]

Thanks @tcrowe, looks fine to me, we need 2 approvals before we can merge so @nivida can you merge when you get a second.

Just a note @tcrowe if your doing doc updates can you please do it against ethereumProvider branch as that will be the new 1.0.x and default branch once its deployed.

[nivida]

@joshstevens19 That's wrong they don't have to be against the ethereumProvider because I didn't change the documentation there and I will merge the ethereumProvider branch into the 1.0 branch.

@tcrowe Thanks for submitting this PR! :)

[joshstevens19]

Surely you want to keep your ethereumProvider inline with the latest code so would of assumed you would want it in the ethereumProvider, sorry if that was incorrect 👍