update typo in line 49 $ cd myapp && tyarn to $ cd myapp && yarn (#647)

Author: brillianodhiya

docs/docs/getting-started.en-US.md

@@ -46,7 +46,7 @@ With umi@3, it's a good template, pro is the basic template, it only provides th
 Install dependencies:
 
 ```shell
-$ cd myapp && tyarn
+$ cd myapp && yarn
 // or
 $ cd myapp && npm install
 ```