XWIKI-15286: XarExtensionExtension has a typo

xwiki-platform-core/pom.xml

...ain/java/org/xwiki/extension/xar/XWikiDocumentMerger.java

...n/java/org/xwiki/extension/xar/XarExtensionException.java

...n/java/org/xwiki/extension/xar/XarExtensionExtension.java

...internal/handler/packager/DefaultXWikiDocumentMerger.java

...iki/extension/xar/internal/handler/packager/Packager.java

...xwiki/extension/xar/script/XarExtensionScriptService.java