remove description and replace cause with source in define_error_type.md

src/error/multiple_error_types/define_error_type.md

@@ -40,11 +40,7 @@ impl fmt::Display for DoubleError {
 
 // This is important for other errors to wrap this one.
 impl error::Error for DoubleError {
- fn description(&self) -> &str {
- "invalid first item to double"
- }
-
- fn cause(&self) -> Option<&error::Error> {
+ fn source(&self) -> Option<&(dyn error::Error + 'static)> {
  // Generic error, underlying cause isn't tracked.
  None
  }