Update README code space and variable declarations

[wozitto]

1. Changed code space from tab to two spaces

The space in the code block has been replaced by a tab from two spaces in the PR here.
When I look at the space with tabs in preview, I feel the space is too wide, so I will change it back to two spaces.

• Current preview

• Preview after changes

2. Changed let declarations to const declarations in the "Dynamic class names with ES2015" example

Since the const declaration has been available from ES2015, I have used it to maintain consistency with other implementation examples.

[dcousens]

This project seems to using tabs typically, but I don't really mind when 2 space is relatively standard in Javascript and this is only documentation

[wozitto]

@dcousens
Indeed, whether it is Tab or 2 space is a trivial difference.
However, since "Real-world example:" uses 2 space, I think it would be better to unify them for the sake of document consistency.
And at least to me it looks cleaner to use 2 space. 👀

[jonkoops]

I think this is a good thing to land, first and foremost because our documentation already uses inconsistent indentation in code examples. Secondarily because be default GitHub indentation for tabs is quite wide.