QImageReader: supportedImageFormats only needs QCoreApplication

Looks like this was an oversight from Qt4 times where there was no
QCoreApplication. But nowadays the plugin loader only needs a
QCoreApplication instead a QApplication.

Change-Id: I878ee157968ca0cd45314f955fc732d0f00b11ab
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 0110819)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/image/qimagereader.cpp

@@ -1489,7 +1489,7 @@ QByteArray QImageReader::imageFormat(QIODevice *device)
     Reading and writing SVG files is supported through the \l{Qt SVG} module.
     The \l{Qt Image Formats} module provides support for additional image formats.
 
-    Note that the QApplication instance must be created before this function is
+    Note that the QCoreApplication instance must be created before this function is
     called.
 
     \sa setFormat(), QImageWriter::supportedImageFormats(), QImageIOPlugin