go_spec: specify that int and uint have the same size

R=r, rsc, iant, ken2
CC=golang-dev
https://golang.org/cl/3992041

doc/go_spec.html

 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of January 7, 2011 -->
+<!-- subtitle Version of January 13, 2011 -->
 
 <!--
 TODO
@@ -699,7 +699,7 @@ There is also a set of predeclared numeric types with implementation-specific si
 
 <pre class="grammar">
 uint     either 32 or 64 bits
-int      either 32 or 64 bits
+int      same size as uint
 float    either 32 or 64 bits
 complex  real and imaginary parts have type float
 uintptr  an unsigned integer large enough to store the uninterpreted bits of a pointer value