spec: adjust complex constant example

Fixes https://golang.org/cl/5444053/#msg41

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5478058

doc/go_spec.html

 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of December 8, 2011 -->
+<!-- subtitle Version of December 10, 2011 -->
 
 <!--
 TODO
@@ -3432,7 +3432,7 @@ const j = true             // j == true  (untyped boolean constant)
 const k = 'w' + 1          // k == 'x'   (untyped character constant)
 const l = "hi"             // l == "hi"  (untyped string constant)
 const m = string(k)        // m == "x"   (type string)
-const Σ = 1 - 0.707        //            (untyped complex constant)
+const Σ = 1 - 0.707i       //            (untyped complex constant)
 const Δ = Σ + 2.0e-4       //            (untyped complex constant)
 const Φ = iota*1i - 1/1i   //            (untyped complex constant)
 </pre>