Doc: fix indexOf QRegularExpression overload documentation

That overload documentation was missing the "exact" qualifier regarding
the search result.

Change-Id: I27a08b60f9b88ae497f8bd9d9ba6a99f4a9ab4d6
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>

src/corelib/tools/qstringlist.cpp

@@ -681,7 +681,7 @@ int QtPrivate::QStringList_lastIndexOf(const QStringList *that, QRegExp &rx, int
     \overload
     \since 5.0
 
-    Returns the index position of the first match of \a re in
+    Returns the index position of the first exact match of \a re in
     the list, searching forward from index position \a from. Returns
     -1 if no item matched.