runtime: fix a minor typo in comments

Change-Id: I13a8aacd1b8243c992b539ab6bf7b5dff2a1393a
Reviewed-on: https://go-review.googlesource.com/1757Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/malloc2.go

@@ -40,7 +40,7 @@ import "unsafe"
 //
 //	3. If the MCentral free list is empty, replenish it by
 //	   allocating a run of pages from the MHeap and then
-//	   chopping that memory into a objects of the given size.
+//	   chopping that memory into objects of the given size.
 //	   Allocating many objects amortizes the cost of locking
 //	   the heap.
 //