spec: typo in switch sample code, missing semicolon

Fixes #697

R=r
CC=golang-dev
https://golang.org/cl/804042

doc/go_spec.html

@@ -3754,7 +3754,7 @@ case 0, 1, 2, 3: s1()
 case 4, 5, 6, 7: s2()
 }
 
-switch x := f() {  // missing switch expression means "true"
+switch x := f(); {  // missing switch expression means "true"
 case x < 0: return -x
 default: return x
 }