doc: add missing space

Author: iamlockon

Diff for: src/subtyping.md

@@ -39,7 +39,7 @@ let subtype: &(dyn for<'a> Fn(&'a i32) -> &'a i32) = &|x| x;
 let supertype: &(dyn Fn(&'static i32) -> &'static i32) = subtype;
 
 // We can also substitute one higher-ranked lifetime for another
-let subtype: &(for<'a, 'b> fn(&'a i32, &'b i32))= &((|x, y| {}) as fn(&_, &_));
+let subtype: &(for<'a, 'b> fn(&'a i32, &'b i32)) = &((|x, y| {}) as fn(&_, &_));
 let supertype: &for<'c> fn(&'c i32, &'c i32) = subtype;
 ```