fix (prismicio-community#152): use trim instead of substr to remove t…

…railing new line

src/Prismic/Dom/RichText.php

@@ -32,7 +32,7 @@ public static function asText($richText)
             }
         }
 
-        return substr($result, 0, -1);
+        return trim($result);
     }
 
     /**