[dv/uvmdvgen] Add comment for testplan

This PR adds a comment to remind user to add testplan to the
`util/build_docs.py` to avoid doc generation error.

Signed-off-by: Cindy Chen <chencindy@google.com>

util/uvmdvgen/index.md.tpl

diff --git a/util/uvmdvgen/index.md.tpl b/util/uvmdvgen/index.md.tpl
index 294fcc4..ee1e88a 100644
--- a/util/uvmdvgen/index.md.tpl
+++ b/util/uvmdvgen/index.md.tpl
@@ -123,5 +123,6 @@
 ```
 
 ${'##'} DV plan
-<!-- TODO: uncomment the line below after adding the testplan -->
+<!-- TODO: uncomment the line below after adding the testplan.
+Please make sure the testplan is added to `/util/build_docs.py`. -->
 {{</* incGenFromIpDesc "hw/ip/${name}/data/${name}_testplan.hjson" "testplan" */>}