doc: develop: testing: Fix reference to test writing section

Add missing colon in :doc: link.

Fixes: fc3283314539 ("doc: Explain briefly how to write new tests")
Signed-off-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/testing.rst b/doc/develop/testing.rst
index 056bdfd..9114d11 100644
--- a/doc/develop/testing.rst
+++ b/doc/develop/testing.rst
@@ -123,7 +123,7 @@
   is much easier to add onto a test - writing a new large test can seem
   daunting to most contributors.
 
-See doc:`tests_writing` for how to write tests.
+See :doc:`tests_writing` for how to write tests.
 
 
 Future work