Doc: Bump import to "QtWebEngine 1.8"

Task-number: QTBUG-70246
Change-Id: I8f56c6ee63fb9b0660558a91c4583961ca0d6946
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/webengine/doc/src/qtwebengine-qmlmodule.qdoc

@@ -26,7 +26,7 @@
 ****************************************************************************/
 
 /*!
-    \qmlmodule QtWebEngine 1.7
+    \qmlmodule QtWebEngine 1.8
     \title Qt WebEngine QML Types
     \brief Provides QML types for rendering web content within a QML application
     \ingroup qtwebengine-modules
@@ -36,7 +36,7 @@
     your .qml file:
 
     \badcode
-    import QtWebEngine 1.7
+    import QtWebEngine 1.8
     \endcode
 
     To link against the module, add the following QT variable to your qmake .pro