Fix variable name mismatch

[TheAlexFuller]

In the code snippet, the variable is called absent_number, but the text refers to some_number. They should be the same name. I went with absent_number from the code.

[carols10cents]

Sorry, but this is correct as-is. There are three variables in the code snippet:

book/listings/ch06-enums-and-pattern-matching/no-listing-06-option-examples/src/main.rs

Lines 3 to 6 in 71352de

	let some_number = Some(5);
	let some_char = Some('e');
	let absent_number: Option<i32> = None;

The first variable is some_number, the second variable is some_char, and the third variable is absent_number. The text addresses each of these in the same order they appear in the code.