encoding/textpb: remove blank line

A blank line between the docstring and the package expression prevents
godoc from recognizing the comment as the package's docstring.

Change-Id: Id3c46ec6dbadb85a9b393c34fe1f0e46ed41a447
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/167766
Reviewed-by: Herbie Ong <herbie@google.com>

encoding/textpb/doc.go

@@ -4,5 +4,4 @@
 
 // Package textpb marshals and unmarshals protocol buffer messages as the
 // textproto format.
-
 package textpb