RangeFrom::step_by docs: fix example
RangeFrom::step_by docs: fix example

The previous example did not do what its description said. In it panicked on integer overflow in debug mode, and went into an infinite loop in release mode (wrapping back to 0 after printing 254).

GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust