doc: use correct snippet id for QObject::isSignalConnected

Copy-and-paste error; the doc was based on QObject::receivers.

Change-Id: I4f6ce716d388328e0345e8457cbc99fff323ce41
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qobject.cpp

@@ -2214,7 +2214,7 @@ int QObject::receivers(const char *signal) const
     \a signal must be a signal member of this object, otherwise the behaviour
     is undefined.
 
-    \snippet code/src_corelib_kernel_qobject.cpp 21
+    \snippet code/src_corelib_kernel_qobject.cpp 49
 
     As the code snippet above illustrates, you can use this function
     to avoid emitting a signal that nobody listens to.