add "(see below)" to a reference to a new concept

This is because I observed someone reading the tutorial who thought they'd
missed something when they got to the mention of variable bindings.

Fixes rust-lang#13570 .

src/doc/tutorial.md

@@ -474,7 +474,7 @@ often convenient to use a block expression for each case, in which case
 the commas are optional as shown below. Literals are valid patterns and
 match only their own value. A single arm may match multiple different
 patterns by combining them with the pipe operator (`|`), so long as every
-pattern binds the same set of variables. Ranges of numeric literal
+pattern binds the same set of variables (see below). Ranges of numeric literal
 patterns can be expressed with two dots, as in `M..N`. The underscore
 (`_`) is a wildcard pattern that matches any single value. (`..`) is a
 different wildcard that can match one or more fields in an `enum` variant.