Updating build repo URL to processing4

The repo URL in the current Processing4 build instructions points to the "old" Processing3 repo.

I figured this out the hard way by accidentally copying the repo URL directly from the build instructions without thinking. Next thing I knew I was getting weird errors saying I need Java 8 to build Processing4. Turns out I was trying to build Processing3...

Author: oseph

build/README.md

@@ -30,13 +30,13 @@ You can choose to install these yourself or use the following guides below:
 One will also need to clone the repository for Processing itself. Some users who are simply building Processing but not contributing to it may prefer a "shallow" clone which does not copy the full history of the repository:
 
 ```
-git clone --depth 1 https://github.com/processing/processing.git
+git clone --depth 1 https://github.com/processing/processing4.git
 ```
 
 Users that are developing for the project may require a full clone:
 
 ```
-git clone https://github.com/processing/processing.git
+git clone https://github.com/processing/processing4.git
 ```
 
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