Add newline at end of JSON.md

GitHub CI is complaining:
```
fix end of files.....................Failed
- hook id: end-of-file-fixer
- exit code: 1
- files were modified by this hook

Fixing doc/JSON.md
[...]
All changes made by hooks:
diff --git a/doc/JSON.md b/doc/JSON.md
index 82170ef..fc7f9cc 100644
[...]
-4.  Within that clone : `make json`
\ No newline at end of file
+4.  Within that clone : `make json`
Error: Process completed with exit code 1.
```

doc/JSON.md

@@ -15,4 +15,4 @@ After Installing the prerequisites
 
 3.  Clone this sail-riscv repository.
 
-4.  Within that clone : `make json`
+4.  Within that clone : `make json`