runtime: fix typo in deferprocStack's comments

change "fp" to "fd".

Change-Id: I00e5fafcb68891356d508c49aa89969bfed7ed10
GitHub-Last-Rev: b06f976a3bacc6fc6f1fd0e4fc8c50da9764f736
GitHub-Pull-Request: golang/go#35557
Reviewed-on: https://go-review.googlesource.com/c/go/+/207038Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/panic.go

@@ -283,7 +283,7 @@ func deferprocStack(d *_defer) {
 	d.varp = 0
 	// The lines below implement:
 	//   d.panic = nil
-	//   d.fp = nil
+	//   d.fd = nil
 	//   d.link = gp._defer
 	//   gp._defer = d
 	// But without write barriers. The first three are writes to