Update README.md

[zacuke]

I know this is a small and inconsequential change, but the spelling error is front and center when a person first looks at the repo.

[zacuke]

Forgive me for reporting misspellings and grammar, but I think I found some in the Wiki too. I don't see a way to create pull requests for the Wiki however.

On page https://github.com/alpacahq/alpaca-trade-api-csharp/wiki/Environments-handling

The word unlimited is misspelled in the 5th paragraph:

• For the Unlimied and Business SIP

And below that this wording seems awkward:

• Version 3.5.0 of SDK also provides a set of helper extension methods for even simplifying code above. For example, you can use a next code snippet for achieving same result:

More appropriate phrasing might be:

• Version 3.5.0 of SDK also provides a set of helper extension methods for simplifying code further. For example, you can use the following code snippet for achieving the same result: