Fix Markdown of links in README

[ejona86]

The []() and [][] styles of linking can't be separated by spaces/newline
on GitHub. I kept the []()-style for short URLs, but for long URLs that
caused line-wrapping issues I swapped to the [][] to make the text more
readable.

[ejona86]

:-/ It seems https://github.com/google/error-prone/wiki/For-Developers and https://github.com/google/error-prone/wiki/Writing-a-check are also broken and could use fixing.

https://github.com/google/error-prone/wiki/Writing-a-check also accidentally uses ````java (instead of ```java; 3 vs 4) prevents it from "closing" the code literal.

[nick-someone]

Thanks for the pointers! I've cleaned up the referenced pages.