playground: Update many variable names for *Calls to call.

In a previous change I renamed many types from *Context to *Call.
In this change I am trying to also update as many variable names
as can easily be changed.  I have only altered variables whose
type was trivially determined.

MultiPart: 6/8

Change-Id: I41760101d0472440b63d39df9833ba97d30274f6
diff --git a/go/src/playground/testdata/src/pong/pong.go b/go/src/playground/testdata/src/pong/pong.go
index c14b606..0b229c2 100644
--- a/go/src/playground/testdata/src/pong/pong.go
+++ b/go/src/playground/testdata/src/pong/pong.go
@@ -16,8 +16,8 @@
 
 type pongd struct{}
 
-func (f *pongd) Ping(ctx ipc.ServerCall, message string) (result string, err error) {
-	remote, _ := ctx.RemoteBlessings().ForCall(ctx)
+func (f *pongd) Ping(call ipc.ServerCall, message string) (result string, err error) {
+	remote, _ := call.RemoteBlessings().ForCall(call)
 	fmt.Printf("%v: %q\n", remote, message)
 	return "PONG", nil
 }