doc: float -> float64 in Effective Go template

R=r, dsymonds
CC=golang-dev
https://golang.org/cl/5530047

doc/effective_go.tmpl

@@ -1490,7 +1490,7 @@ 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" }