update existing references to yarn dev to use yarn start instead

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/plugins/create-a-plugin.md

@@ -25,7 +25,7 @@ And then select `frontend-plugin`.
 This will create a new Backstage Plugin based on the ID that was provided. It
 will be built and added to the Backstage App automatically.
 
-> If the Backstage App is already running (with `yarn start` or `yarn dev`) you
+> If the Backstage App is already running (with `yarn start`) you
 > should be able to see the default page for your new plugin directly by
 > navigating to `http://localhost:3000/my-plugin`.
 

docs/plugins/integrating-search-into-plugins.md

@@ -245,7 +245,7 @@ You can also check out the documentation on [how to test Backstage plugin module
 
 #### 9. Running the collator locally
 
-Run `yarn dev` in the root folder of your Backstage project and look for logs like these:
+Run `yarn start` in the root folder of your Backstage project and look for logs like these:
 
 ```sh
 [backend]: YYYY-MM-DDTHH:MM:SS.000Z search info Task worker starting: search_index_faq_snippets, {"version":2,"cadence":"PT10M","initialDelayDuration":"PT3S","timeoutAfterDuration":"PT15M"} task=search_index_faq_snippets