Typo fix in use declaration section of reference

Author: therustmonk

src/doc/reference.md

@@ -881,7 +881,7 @@ mod foo {
     }
 
     use foo::example::iter; // good: foo is at crate root
-//  use example::iter;      // bad:  core is not at the crate root
+//  use example::iter;      // bad:  example is not at the crate root
     use self::baz::foobaz;  // good: self refers to module 'foo'
     use foo::bar::foobar;   // good: foo is at crate root