minor #17086 CS: use nowdoc instead of heredoc (gharlan)

This PR was merged into the 2.3 branch.

Discussion
----------

CS: use nowdoc instead of heredoc

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | —
| License       | MIT
| Doc PR        | —

if this is accepted, we could add the fixer to symfony level of php-cs-fixer: PHP-CS-Fixer/PHP-CS-Fixer#1580

Commits
-------

3dca549 use nowdoc instead of heredoc

Author: nicolas-grekas

Tests/ConstraintViolationListTest.php

@@ -110,7 +110,7 @@ public function testToString()
             $this->getViolation('Error 5', '', '[baz]'),
         ));
 
-        $expected = <<<EOF
+        $expected = <<<'EOF'
 Root:
     Error 1
 Root.foo.bar:

Tests/ConstraintViolationTest.php

@@ -26,7 +26,7 @@ public function testToStringHandlesArrays()
             null
         );
 
-        $expected = <<<EOF
+        $expected = <<<'EOF'
 Root.property.path:
     Array
 EOF;
@@ -45,7 +45,7 @@ public function testToStringHandlesArrayRoots()
             null
         );
 
-        $expected = <<<EOF
+        $expected = <<<'EOF'
 Array.some_value:
     42 cannot be used here
 EOF;