runtime: fix getcallersp documentation
LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/180760043
diff --git a/src/runtime/stubs.go b/src/runtime/stubs.go
index fe8f9c9..1282397 100644
--- a/src/runtime/stubs.go
+++ b/src/runtime/stubs.go
@@ -231,7 +231,7 @@
//
// func f(arg1, arg2, arg3 int) {
// pc := getcallerpc(unsafe.Pointer(&arg1))
-// sp := getcallerpc(unsafe.Pointer(&arg2))
+// sp := getcallersp(unsafe.Pointer(&arg1))
// }
//
// These two lines find the PC and SP immediately following
