Remove 1-ary space

This follows the formatting standards of rustfmt and rustc.

Author: ehuss

src/types/tuple.md

@@ -27,7 +27,7 @@ Its one value is also called *unit* or *the unit value*.
 Some examples of tuple types:
 
 * `()` (unit)
-* `(i32, )` (1-ary tuple)
+* `(i32,)` (1-ary tuple)
 * `(f64, f64)`
 * `(String, i32)`
 * `(i32, String)` (different type from the previous example)