minor #2772 [Map] Add return type to __toString() (dannyvw)

This PR was merged into the 2.x branch.

Discussion
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[Map] Add return type to __toString()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Docs?         | no <!-- required for new features -->
| Issues        | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

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Commits
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dcd2426 [Map] Add return type to __toString()

Author: Kocal

src/Map/src/Renderer/Renderers.php

@@ -57,7 +57,7 @@ public function renderMap(Map $map, array $attributes = []): string
         return $renderer->renderMap($map, $attributes);
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return implode(', ', array_keys($this->renderers));
     }