doc: fix invalid id attribute in faq

R=r
CC=golang-dev
https://golang.org/cl/3516042

doc/go_faq.html

@@ -661,7 +661,7 @@ See the <a href="/doc/effective_go.html#allocation_new">relevant section
 of Effective Go</a> for more details.
 </p>
 
-<h3 id="64bit_machine_32bit_int">
+<h3 id="q_int_sizes">
 Why is <code>int</code> 32 bits on 64 bit machines?</h3>
 
 <p>