Fix comma in debug_assert! docs

Author: pierwill

src/libcore/macros/mod.rs

@@ -151,7 +151,7 @@ macro_rules! assert_ne {
 /// An unchecked assertion allows a program in an inconsistent state to keep
 /// running, which might have unexpected consequences but does not introduce
 /// unsafety as long as this only happens in safe code. The performance cost
-/// of assertions, is however, not measurable in general. Replacing [`assert!`]
+/// of assertions, however, is not measurable in general. Replacing [`assert!`]
 /// with `debug_assert!` is thus only encouraged after thorough profiling, and
 /// more importantly, only in safe code!
 ///