A typo in README.md: "a accessor" should be "an accessor"

[shshsh-sh]

I found a minor typo in the README.md documentation.

In At a Glance section, the comment explaining createSignal:

Current text:

Create a piece of reactive state, giving us a accessor, count(), and a setter, setCount()

Suggested fix:

Create a piece of reactive state, giving us an accessor, count(), and a setter, setCount()

"accessor" starts with a vowel sound (the 'a' in "accessor" is pronounced as a vowel), so it should be preceded by "an" rather than "a" according to English grammar rules.

This is a very small issue but fixing it would improve the grammatical correctness of the documentation.

I have fixed this typo and I will make a PR.