Fix warning in str_replace().

src/Plugin/views/area/ViewsFiltersSummary.php

@@ -255,7 +255,7 @@ class ViewsFiltersSummary extends Result {
           '#markup' => str_replace(
             array_keys($replacements),
             array_values($replacements),
-            $this->options['content']
+            (string) $this->options['content']
           ),
         ];
       }