logging: remove godoc about non-existent log.Logger interface

The previous wording implied that the standard library's
*log.Logger is an interface, yet it is a concrete type.
Also the comment in the example used
    "implements"
but should have been
    "is an instance of"

Fixes #1508

Change-Id: Ibbb6a63edea4f51307521516f20a9976786a8435
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/42872
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/logging/doc.go b/logging/doc.go
index f63d499..2a2a4ce 100644
--- a/logging/doc.go
+++ b/logging/doc.go
@@ -85,11 +85,11 @@
 	lg.Log(logging.Entry{Payload: json.RawMessage(j)})
 
 
-The Standard Logger Interface
+The Standard Logger
 
 You may want use a standard log.Logger in your program.
 
-	// stdlg implements log.Logger
+	// stdlg is an instance of *log.Logger.
 	stdlg := lg.StandardLogger(logging.Info)
 	stdlg.Println("some info")