doc: use the new log functions

The log interface was changed in https://golang.org/cl/2419042.

Change-Id: I3eaddd8a5cfcae961db16555fb1b0ce6770b6334
Reviewed-on: https://go-review.googlesource.com/c/143777
Reviewed-by: Rob Pike <r@golang.org>

doc/effective_go.html

@@ -2762,15 +2762,15 @@ <h2 id="embedding">Embedding</h2>
 }
 </pre>
 <p>
-The <code>Job</code> type now has the <code>Log</code>, <code>Logf</code>
+The <code>Job</code> type now has the <code>Print</code>, <code>Printf</code>, <code>Println</code>
 and other
 methods of <code>*log.Logger</code>.  We could have given the <code>Logger</code>
 a field name, of course, but it's not necessary to do so.  And now, once
 initialized, we can
 log to the <code>Job</code>:
 </p>
 <pre>
-job.Log("starting now...")
+job.Println("starting now...")
 </pre>
 <p>
 The <code>Logger</code> is a regular field of the <code>Job</code> struct,
@@ -2797,8 +2797,8 @@ <h2 id="embedding">Embedding</h2>
 which would be useful if we wanted to refine the methods of <code>Logger</code>.
 </p>
 <pre>
-func (job *Job) Logf(format string, args ...interface{}) {
-    job.Logger.Logf("%q: %s", job.Command, fmt.Sprintf(format, args...))
+func (job *Job) Printf(format string, args ...interface{}) {
+    job.Logger.Printf("%q: %s", job.Command, fmt.Sprintf(format, args...))
 }
 </pre>
 <p>