Update restriction on declarations of methods.

APPROVED=rsc
DELTA=1  (0 added, 0 deleted, 1 changed)
OCL=34289
CL=34289

doc/go_spec.html

@@ -1659,7 +1659,7 @@ The receiver type must be of the form <code>T</code> or <code>*T</code> where
 <code>T</code> is a type name. <code>T</code> is called the
 <i>receiver base type</i> or just <i>base type</i>.
 The base type must not be a pointer or interface type and must be
-declared in the same source file as the method.
+declared in the same package as the method.
 The method is said to be <i>bound</i> to the base type
 and is visible only within selectors for that type
 (§<a href="#Type_declarations">Type declarations</a>, §<a href="#Selectors">Selectors</a>).