doc: add a missing space in func signature

Fixes #11383

Change-Id: I7ef0bb8b535ef513afdfcda19317ae4ff21f7166
Reviewed-on: https://go-review.googlesource.com/11490
Reviewed-by: Josh Bleecher Snyder <[email protected]>

Author: bradfitz

doc/effective_go.html

@@ -1421,7 +1421,7 @@ <h3 id="slices">Slices</h3>
 <code>nil</code> slice, and return 0.
 </p>
 <pre>
-func Append(slice, data[]byte) []byte {
+func Append(slice, data []byte) []byte {
     l := len(slice)
     if l + len(data) &gt; cap(slice) {  // reallocate
         // Allocate double what's needed, for future growth.