Add how to fix for PossiblyUndefinedArrayOffset

docs/running_psalm/issues/PossiblyUndefinedArrayOffset.md

@@ -10,5 +10,24 @@ if (rand(0, 1)) {
 }
 
 echo $arr["b"];
+```
+
+## How to fix
+
+You can use the null coalesce operator to provide a default value in the event the array offset doesn’t exist:
 
 ```
+echo $arr["b"] ?? 0;
+```
+
+Alternatively, you can ensure that the array offset always exists:
+
+```php
+if (rand(0, 1)) {
+    $arr = ["a" => 1, "b" => 2];
+} else {
+    $arr = ["a" => 3, "b" => 0];
+}
+
+echo $arr["b"];
+```