Fix spelling issues in documentation

[mdqst]

What was the problem?

I noticed a couple of small spelling errors in the documentation that I’ve gone ahead and corrected:

1. In the docs/docs-user/exchanges.md file, "Houbi" has been corrected to "Huobi".
2. In the docs/docs-user/wallets.md file, "Mutisignature" has been corrected to "Multisignature" in the "Safe" section.

Thanks for Lisk!