Docs: Fix the snippet lookup for QFileDialog::getOpenFileContent

src_gui_dialogs_qfiledialog.cpp had two snippets labeled "14". This
change bumps one of them to "15" and fixes the the snippet lookup for
QFileDialog::getOpenFileContent accordingly.

Also, fix two typos:
"QSting" -> "QString" and "contents has" -> "contents have".

Change-Id: Ic018c23b6ca585f30c116b8a6eb29293560c7a35
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/widgets/dialogs/qfiledialog.cpp

@@ -2376,10 +2376,10 @@ QList<QUrl> QFileDialog::getOpenFileUrls(QWidget *parent,
     It can also be used on other platforms, where it will fall back to using QFileDialog.
 
     The function is asynchronous and returns immediately. The \a fileOpenCompleted
-    callback will be called when a file has been selected and its contents has been
+    callback will be called when a file has been selected and its contents have been
     read into memory.
 
-    \snippet code/src_gui_dialogs_qfiledialog.cpp 14
+    \snippet code/src_gui_dialogs_qfiledialog.cpp 15
     \since 5.13
 */
 void QFileDialog::getOpenFileContent(const QString &nameFilter, const std::function<void(const QString &, const QByteArray &)> &fileOpenCompleted)

src/widgets/doc/snippets/code/src_gui_dialogs_qfiledialog.cpp

@@ -145,8 +145,8 @@ dialog.exec();
 "Images (*.png *.xpm *.jpg);;Text files (*.txt);;XML files (*.xml)"
 //! [14]
 
-//! [14]
-auto fileOpenCompleted = [](const QSting &fileName, const QByteArray &fileContent) {
+//! [15]
+auto fileOpenCompleted = [](const QString &fileName, const QByteArray &fileContent) {
     if (fileName.isEmpty()) {
         // No file was selected
     } else {
@@ -154,4 +154,4 @@ auto fileOpenCompleted = [](const QSting &fileName, const QByteArray &fileConten
     }
 }
 QFileDialog::getOpenFileContent("Images (*.png *.xpm *.jpg)",  fileContentReady);
-//! [14]
+//! [15]