Fix installation instruction

Author: mfn

README.md

@@ -18,7 +18,7 @@ generated by ANTLR4. The runtime provides a common set of tools for using your p
 Install the runtime with Composer:
 
 ```bash
-composer require antlr4-php-runtime
+composer require antlr/antlr4-php-runtime
 ```
 
 #### 3. Generate your parser