debug/macho: fix comment on Section64

Change-Id: I7c809ec385b56ebb2ec784a1479d466df6ab4d1a
Reviewed-on: https://go-review.googlesource.com/27565Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/macho/macho.go

@@ -145,7 +145,7 @@ type Section32 struct {
 	Reserve2 uint32
 }
 
-// A Section32 is a 64-bit Mach-O section header.
+// A Section64 is a 64-bit Mach-O section header.
 type Section64 struct {
 	Name     [16]byte
 	Seg      [16]byte