debug/gosym: fix typo in comment

Change-Id: Ieb13359c5bbe26bbf7baaaa8eb63d5e90bdefdd8
Reviewed-on: https://go-review.googlesource.com/7591Reviewed-by: Minux Ma <minux@golang.org>

src/debug/gosym/symtab.go

@@ -30,7 +30,7 @@ type Sym struct {
 	Type   byte
 	Name   string
 	GoType uint64
-	// If this symbol if a function symbol, the corresponding Func
+	// If this symbol is a function symbol, the corresponding Func
 	Func *Func
 }