add a line to the example to clarify semantics

This is to clarify that match construct doesn't define a new variable, since I
observed a person reading the Rust tutorial who seemed to incorrectly think
that it did. Fixes #13571 .

Author: Zooko Wilcox-O'Hearn

src/doc/tutorial.md

@@ -455,7 +455,7 @@ against each pattern in order until one matches. The matching pattern
 executes its corresponding arm.
 
 ~~~~
-# let my_number = 1;
+let my_number = 1;
 match my_number {
   0     => println!("zero"),
   1 | 2 => println!("one or two"),