Avoid calling a variable 'string' when discussing strings.

Newcomers to Rust need to learn the distinctinion between `&str` and
`String`, so additonally having `string` in an example risks confusion.

Author: Wilfred

src/doc/trpl/strings.md

@@ -16,7 +16,7 @@ Rust has two main types of strings: `&str` and `String`. Let’s talk about
 `&'static str`:
 
 ```rust
-let string = "Hello there."; // string: &'static str
+let greeting = "Hello there."; // greeting: &'static str
 ```
 
 This string is statically allocated, meaning that it’s saved inside our