doc: only the Logger.SetOutput method is new in Go 1.5

The SetOutput function has been there since Go 1.

Fixes #12162.

Change-Id: I66210374877581e42689f9943532141659a55ca7
Reviewed-on: https://go-review.googlesource.com/13637Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.html

@@ -1087,9 +1087,8 @@ uses a caller-provided buffer, permitting control of allocation and buffer size.
 The <a href="/pkg/log/"><code>log</code></a> package
 has a new <a href="/pkg/log/#LUTC"><code>LUTC</code></a> flag
 that causes time stamps to be printed in the UTC time zone.
-It also adds a <a href="/pkg/log/#SetOutput"><code>SetOutput</code></a> function
-to set the output destination for the standard logger
-and a corresponding method for user-created loggers.
+It also adds a <a href="/pkg/log/#Logger.SetOutput"><code>SetOutput</code></a> method
+for user-created loggers.
 </li>
 
 <li>