Update soliditySha3 docs

[spacesailor24]

Fixes #3907

[coveralls]

Pull Request Test Coverage Report for Build 552354768

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 226 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+2.5%) to 76.273%

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Files with Coverage Reduction	New Missed Lines	%
packages/web3-core-requestmanager/src/jsonrpc.js	1	70.0%
packages/web3-core-helpers/src/formatters.js	8	82.27%
packages/web3-utils/src/utils.js	27	10.74%
packages/web3-core-helpers/src/errors.js	29	1.56%
packages/web3-utils/src/soliditySha3.js	34	3.92%
packages/web3-utils/src/index.js	42	32.12%
packages/web3-eth-accounts/src/index.js	85	33.06%

Totals
Change from base Build 538231133:	2.5%
Covered Lines:	3172
Relevant Lines:	3944

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💛 - Coveralls

[koraykoska]

Totally something which should be fixed in terms of best practices. We should even think about deprecating it. I just tested it and the examples are correct.