Add erroneous code example for E0131

src/librustc_typeck/diagnostics.rs

@@ -1721,6 +1721,12 @@ extern {
 E0131: r##"
 It is not possible to define `main` with type parameters, or even with function
 parameters. When `main` is present, it must take no arguments and return `()`.
+Erroneous code example:
+
+```
+fn main<T>() { // error: main function is not allowed to have type parameters
+}
+```
 "##,
 
 E0132: r##"