Change nil to bson.D in documentation examples.

GODRIVER-757

Change-Id: Ice9647faaedc235d6ae6915c2a7d46060fa0a461

Author: Divjot Arora

README.md

@@ -85,7 +85,7 @@ Several query methods return a cursor, which can be used like this:
 
 ```go
 ctx, _ = context.WithTimeout(context.Background(), 30*time.Second)
-cur, err := collection.Find(ctx, nil)
+cur, err := collection.Find(ctx, bson.D{})
 if err != nil { log.Fatal(err) }
 defer cur.Close(ctx)
 for cur.Next(ctx) {

mongo/doc.go

@@ -31,7 +31,7 @@
 //
 // Several methods return a cursor, which can be used like this:
 //
-//    cur, err := collection.Find(context.Background(), nil)
+//    cur, err := collection.Find(context.Background(), bson.D{})
 //    if err != nil { log.Fatal(err) }
 //    defer cur.Close(context.Background())
 //    for cur.Next(context.Background()) {