Fix comment for Buffer.index field PiperOrigin-RevId: 130332792

diff --git a/proto/lib.go b/proto/lib.go
index 170b8e8..ac4ddbc 100644
--- a/proto/lib.go
+++ b/proto/lib.go

@@ -308,7 +308,7 @@
 // temporary Buffer and are fine for most applications.
 type Buffer struct {
 	buf   []byte // encode/decode byte stream
-	index int    // write point
+	index int    // read point
 
 	// pools of basic types to amortize allocation.
 	bools   []bool