runtime: fix comments style typo

Code comments should have a space between comments characters and
actual words.

Change-Id: I6274baf1fc09b37a32ec6c69ddbb8edca9eb5469
Reviewed-on: https://go-review.googlesource.com/120475Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/iface.go

@@ -18,7 +18,7 @@ var (
 	itabTableInit = itabTableType{size: itabInitSize} // starter table
 )
 
-//Note: change the formula in the mallocgc call in itabAdd if you change these fields.
+// Note: change the formula in the mallocgc call in itabAdd if you change these fields.
 type itabTableType struct {
 	size    uintptr             // length of entries array. Always a power of 2.
 	count   uintptr             // current number of filled entries.