remove trailing whitespace

Author: nikomatsakis

src/test/compile-fail/for-loop-unconstrained-element-type.rs

@@ -11,7 +11,7 @@
 // Test that `for` loops don't introduce artificial
 // constraints on the type of the binding (`i`).
 // Subtle changes in the desugaring can cause the
-// type of elements in the vector to (incorrectly) 
+// type of elements in the vector to (incorrectly)
 // fallback to `!` or `()`.
 
 fn main() {