minor #11095 [2.3][Process] Minor README update (romainneutron)

This PR was merged into the 2.3 branch.

Discussion
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[2.3][Process] Minor README update

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Commits
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c7d6bbb [Process] Minor README update

src/Symfony/Component/Process/README.md

@@ -27,7 +27,7 @@ as it becomes available:
 
     $process = new Process('ls -lsa');
     $process->run(function ($type, $buffer) {
-        if ('err' === $type) {
+        if (Process::ERR === $type) {
             echo 'ERR > '.$buffer;
         } else {
             echo 'OUT > '.$buffer;
@@ -42,6 +42,6 @@ Resources
 
 You can run the unit tests with the following command:
 
-    $ cd path/to/Symfony/Component/XXX/
+    $ cd path/to/Symfony/Component/Process/
     $ composer.phar install
     $ phpunit