Real Errors Identified in Calculation by Scrutinizer (#2775)

Before Scrutinizer broke, I took a look at the remaining 43 errors which it categorized as 'major'. Most of these were false positives, but, in the case of Calculation and Reader/Xlsx/Chart, I was able to determine that its analysis of some of the problems was correct. There is little point addressing the false positives until it starts working again, but we should fix the real errors.

This PR addresses the real errors in Reader/Xlsx/Chart.
- An assignment to `$XaxisLable` should be an assignment to `$XaxisLabel`.
- An assignment to `$YaxisLable` should be an assignment to `$YaxisLabel`.

src/PhpSpreadsheet/Reader/Xlsx/Chart.php

@@ -71,7 +71,7 @@ class Chart
                         $chartDetailsC = $chartDetails->children($namespacesChartMeta['c']);
                         switch ($chartDetailsKey) {
                             case 'plotArea':
-                                $plotAreaLayout = $XaxisLable = $YaxisLable = null;
+                                $plotAreaLayout = $XaxisLabel = $YaxisLabel = null;
                                 $plotSeries = $plotAttributes = [];
                                 foreach ($chartDetails as $chartDetailKey => $chartDetail) {
                                     switch ($chartDetailKey) {