Revert "Rename string.adoc to String.adoc"

This reverts commit e31bf92.

In Arduino, the word string refers to two different things:
- Instance of the String class: Called a "String".
- Null terminated char array: Called a "string".

Since they are two very different things that only differ in the capitalization of the first letter, this causes a lot of confusion for beginners. Thus, it's very important that the reference correctly capitalize these words.

Author: per1234