effective_go: remove unused variable from a couple of examples

Fixes #2481.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5435068

doc/effective_go.html

@@ -2046,7 +2046,7 @@ It's easy to write a function to print the arguments.
 </p>
 <pre>
 func ArgServer() {
-    for i, s := range os.Args {
+    for _, s := range os.Args {
         fmt.Println(s)
     }
 }
@@ -2084,7 +2084,7 @@ to have the right signature.
 <pre>
 // Argument server.
 func ArgServer(w http.ResponseWriter, req *http.Request) {
-    for i, s := range os.Args {
+    for _, s := range os.Args {
         fmt.Fprintln(w, s)
     }
 }

doc/effective_go.tmpl

@@ -1984,7 +1984,7 @@ It's easy to write a function to print the arguments.
 </p>
 <pre>
 func ArgServer() {
-    for i, s := range os.Args {
+    for _, s := range os.Args {
         fmt.Println(s)
     }
 }
@@ -2022,7 +2022,7 @@ to have the right signature.
 <pre>
 // Argument server.
 func ArgServer(w http.ResponseWriter, req *http.Request) {
-    for i, s := range os.Args {
+    for _, s := range os.Args {
         fmt.Fprintln(w, s)
     }
 }