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/qapi/trace-events b/qapi/trace-events
index 3cabe91..cccafc0 100644
--- a/qapi/trace-events
+++ b/qapi/trace-events
@@ -1,4 +1,4 @@
-# See docs/devel/tracing.txt for syntax documentation.
+# See docs/devel/tracing.rst for syntax documentation.
 
 # qapi-visit-core.c
 visit_free(void *v) "v=%p"