Fix string interpolation of newline

src/Monolog/Formatter/LineFormatter.php

@@ -155,7 +155,7 @@ class LineFormatter extends NormalizerFormatter
             do {
                 $depth++;
                 if ($depth > $this->maxNormalizeDepth) {
-                    $str .= '\n[previous exception] Over ' . $this->maxNormalizeDepth . ' levels deep, aborting normalization';
+                    $str .= "\n[previous exception] Over " . $this->maxNormalizeDepth . ' levels deep, aborting normalization';
                     break;
                 }