net/textproto: delete spurious quote

Sigh.

R=golang-dev, gri, nigeltao
CC=golang-dev
https://golang.org/cl/5675071

src/pkg/net/textproto/textproto.go

@@ -21,7 +21,7 @@
 // Writer, to write dot-encoded text blocks.
 //
 // Conn, a convenient packaging of Reader, Writer, and Pipeline for use
-// with a single network connection."
+// with a single network connection.
 //
 package textproto