Rollup merge of rust-lang#30020 - Manishearth:unit, r=bluss

`unit` was in code formatting, which is wrong, since it's not actual code. The correct code is `()`.

Author: steveklabnik

src/doc/reference.md

@@ -677,8 +677,8 @@ apply to the crate as a whole.
 ```
 
 A crate that contains a `main` function can be compiled to an executable. If a
-`main` function is present, its return type must be [`unit`](#tuple-types)
-and it must take no arguments.
+`main` function is present, its return type must be `()`
+("[unit](#tuple-types)") and it must take no arguments.
 
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