Fix path in Accessiblity docs

I'm not sure if this should be here in the first place, but at least
point to a valid location.

Change-Id: I72342fc53ba2ec3fc0a711c62567f63883a23f37
Reviewed-by: Martin Smith <martin.smith@qt.io>

doc/src/frameworks-technologies/accessible.qdoc

@@ -356,7 +356,7 @@
     in turn inherits QAccessibleWidget. You do not need to examine the
     QAccessibleAbstractSlider class to read this section. If you want
     to take a look, the code for all of Qt's accessible interfaces are
-    found in src/plugins/accessible/widgets. Here is the
+    found in qtbase/src/widgets/accessible. Here is the
     QAccessibleSlider's constructor:
 
     \snippet snippets/accessibilityslidersnippet.cpp 0