Clean up E0648 explanation

src/librustc_error_codes/error_codes/E0648.md

-`export_name` attributes may not contain null characters (`\0`).
+An `export_name` attribute contains null characters (`\0`).
+
+Erroneous code example:
 
 ```compile_fail,E0648
 #[export_name="\0foo"] // error: `export_name` may not contain null characters
 pub fn bar() {}
 ```
+
+To fix this error, remove the null characters:
+
+```
+#[export_name="foo"] // ok!
+pub fn bar() {}
+```