Clean up E0131 error explanation

src/librustc_error_codes/error_codes/E0131.md

-It is not possible to define `main` with generic parameters.
-When `main` is present, it must take no arguments and return `()`.
+The `main` function was defined with generic parameters.
+
 Erroneous code example:
 
 ```compile_fail,E0131
 fn main<T>() { // error: main function is not allowed to have generic parameters
 }
 ```
+
+It is not possible to define the `main` function with generic parameters.
+It must not take any arguments.