update readme at sec Build & Run

Author: scPointer

README.md

@@ -31,8 +31,8 @@
 $ git clone https://github.com/LearningOS/rCore-Tutorial-Code-2023S.git
 $ cd rCore-Tutorial-Code-2023S
 $ git clone https://github.com/LearningOS/rCore-Tutorial-Test-2023S.git user
-$ cd os
 $ git checkout ch$ID
+$ cd os
 # run OS in ch$ID
 $ make run
 ```
@@ -51,4 +51,4 @@ $ git checkout ch$ID
 # check&grade OS in ch$ID with more tests
 $ cd ci-user && make test CHAPTER=$ID
 ```
-Notice: $ID is from [3,4,5,6,8]
+Notice: $ID is from [3,4,5,6,8]