doc: float -> float64 in Effective Go

R=golang-dev
CC=golang-dev
https://golang.org/cl/5531046

doc/effective_go.html

@@ -1494,7 +1494,7 @@ format <code>%#v</code> prints the value in full Go syntax.
 <pre>
 type T struct {
     a int
-    b float
+    b float64
     c string
 }
 t := &amp;T{ 7, -2.35, "abc\tdef" }