tutorial: Fix formatting

Author: brson

doc/tutorial.md

@@ -216,7 +216,7 @@ while count < 10 {
 
 The name of the function that prints a line of text, `io::println`, is
 qualified: it refers to the function named `println` that's defined in the
-module `io`. In Rust, a double colon---`::`---separates parts of a
+module `io`. In Rust, a double colon separates parts of a
 qualified name. For more details, see the section on [crates](#crates).
 
 Although Rust can almost always infer the types of local variables, you