Fix docs example for impurity

docs/running_psalm/issues.md

@@ -337,9 +337,20 @@ takesString(new A);
 Emitted when calling an impure function from a function or method marked as pure.
 
 ```php
+function impure(array $a) : array {
+    /** @var int */
+    static $i = 0;
+
+    ++$i;
+
+    $a[$i] = 1;
+
+    return $a;
+}
+
 /** @psalm-pure */
 function filterOdd(array $a) : void {
-    extract($a);
+    impure($a);
 }
 ```