fix namespace at Example on README (#3)

Author: sasezaki

README.md

@@ -20,7 +20,7 @@ If you need to set a pre-processor define, you can use a context;
 ```php
 $parser = new PHPCParser\CParser;
 
-$context = new PHPCParser\PreProcessor\Context;
+$context = new PHPCParser\Context;
 // #define A 42
 $context->defineInt('A', 42);
 // #define B "testing"
@@ -38,4 +38,4 @@ And that's all there is to it (until it is workting that is...)...
 
 ```console
 $ clang -cc1 -ast-dump test.c
-```
+```