Doc: Show correct headerfile for StringLiterals

The automatically generated `#include` statement for the
`Qt::Literals::StringLiterals` namespace contains the wrong header file
name. Use QDoc's `\headerfile` command to ensure QDoc generates the
correct `#include` statement.

Pick-to: 6.5
Fixes: QTBUG-112884
Change-Id: I825402dc9d8d451fecc120697486d4bf81e468d3
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/text/qtliterals.qdoc

@@ -11,6 +11,7 @@
 /*!
     \namespace Qt::Literals
     \inmodule QtCore
+    \headerfile QtLiterals
 
     \brief The Literals inline namespace declares literal operators for Qt types.
 */
@@ -18,6 +19,7 @@
 /*!
     \namespace Qt::Literals::StringLiterals
     \inmodule QtCore
+    \headerfile QStringLiterals
 
     \brief The StringLiterals namespace declares string literal operators
     for Qt types.