[MINOR] Fix instructions on how to build Livy.

## What changes were proposed in this pull request?
Update the README section with instructions on building Livy 

## How was this patch tested?
- Checked the rendered markdown.
- Executed the instructions.

## What remains to be done?
This instruction should be updated in the documentation each time Livy's repository name changes.

README.md

@@ -67,7 +67,7 @@ Livy is built using [Apache Maven](http://maven.apache.org). To check out and bu
 
 ```
 git clone https://github.com/apache/incubator-livy.git
-cd livy
+cd incubator-livy
 mvn package
 ```