Fix minor error in chain-spec generation command in readme (#1644)

* Fix minor error in chainspec generation command in readme

* Fixed up remaining typo

substrate/README.adoc

@@ -132,7 +132,7 @@ First let's get a template chainspec that you can edit. We'll use the "staging"
 
 [source, shell]
 ----
-substrate --chain=staging build-spec > ~/chainspec.json
+substrate build-spec --chain=staging > ~/chainspec.json
 ----
 
 Now, edit `~/chainspec.json` in your editor. There are a lot of individual fields for each module, and one very large one which contains the Webassembly code blob for this chain. The easiest field to edit is the block `period`. Change it to 10 (seconds):