docs: fix references to docs/devel/tracing.rst

Commit e50caf4a5c ("tracing: convert documentation to rST")
converted docs/devel/tracing.txt to docs/devel/tracing.rst.

We still have several references to the old file, so let's fix them
with the following command:

  sed -i s/tracing.txt/tracing.rst/ $(git grep -l docs/devel/tracing.txt)

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210517151702.109066-2-sgarzare@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
diff --git a/hw/usb/trace-events b/hw/usb/trace-events
index 38e05fc..b8287b6 100644
--- a/hw/usb/trace-events
+++ b/hw/usb/trace-events
@@ -1,4 +1,4 @@
-# See docs/devel/tracing.txt for syntax documentation.
+# See docs/devel/tracing.rst for syntax documentation.
 
 # core.c
 usb_packet_state_change(int bus, const char *port, int ep, void *p, const char *o, const char *n) "bus %d, port %s, ep %d, packet %p, state %s -> %s"