net: add missing period in conn.File documentation

Change-Id: Ie873d7ed595c91cee4c1aa6c22fa44b61b6190ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/169138
Run-TryBot: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/net.go

@@ -282,7 +282,7 @@ func (c *conn) SetWriteBuffer(bytes int) error {
 	return nil
 }
 
-// File returns a copy of the underlying os.File
+// File returns a copy of the underlying os.File.
 // It is the caller's responsibility to close f when finished.
 // Closing c does not affect f, and closing f does not affect c.
 //