doc: tweak example in Effective Go

A prior attempt at addressing the issue got bogged down in an
endless conversation around the subtleties of Read semantics.
Let's not go there.

Instead, we put the issue to bed, perhaps not in perfect comfort
but well enough, by moving a line of the example so that even
if there is a "benign" error as the issue suggests, the loop
terminates with n and err correctly set.

Fixes #27818

Change-Id: I4a32d56c9e782f17578565d90b22ce531e3d8667
Reviewed-on: https://go-review.googlesource.com/c/143677
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/effective_go.html

@@ -1402,11 +1402,11 @@ the moment, the following snippet would also read the first 32 bytes of the buff
     var err error
     for i := 0; i < 32; i++ {
         nbytes, e := f.Read(buf[i:i+1])  // Read one byte.
+        n += nbytes
         if nbytes == 0 || e != nil {
             err = e
             break
         }
-        n += nbytes
     }
 </pre>
 <p>