runtime: fix stale comment in lfstack

Change-Id: I6ef08f6078190dc9df0b2df4f26a76456602f5e8
Reviewed-on: https://go-review.googlesource.com/24176
Reviewed-by: Rick Hudson <[email protected]>

Author: aclements

src/runtime/lfstack.go

@@ -6,7 +6,7 @@
 // Initialize head to 0, compare with 0 to test for emptiness.
 // The stack does not keep pointers to nodes,
 // so they can be garbage collected if there are no other pointers to nodes.
-// The following code runs only on g0 stack.
+// The following code runs only in non-preemptible contexts.
 
 package runtime