docs: Name strategy regexp in example need to be corrected (argoproj-labs#534)

Name strategy among Update strategies, have its example have a wrong regexp. Need correction to match format mentioned in last line `YYYY-MM-DD`. 
Current one is this format YYYY-MMDD (which is not what, doc wish to show)

Author: iamraj007

docs/basics/update-strategies.md

@@ -278,15 +278,15 @@ for update, you will need additional configuration. For example,
 ```yaml
 argocd-image-updater.argoproj.io/image-list: myimage=some/image
 argocd-image-updater.argoproj.io/myimage.update-strategy: name
-argocd-image-updater.argoproj.io/myimage.allow-tags: regexp:^[0-9]{4}-[0-9]{2}[0-9]{2}$
+argocd-image-updater.argoproj.io/myimage.allow-tags: regexp:^[0-9]{4}-[0-9]{2}-[0-9]{2}$
 ```
 
 or 
 
 ```yaml
 argocd-image-updater.argoproj.io/image-list: myimage=some/image
 argocd-image-updater.argoproj.io/myimage.update-strategy: alphabetical
-argocd-image-updater.argoproj.io/myimage.allow-tags: regexp:^[0-9]{4}-[0-9]{2}[0-9]{2}$
+argocd-image-updater.argoproj.io/myimage.allow-tags: regexp:^[0-9]{4}-[0-9]{2}-[0-9]{2}$
 ```
 
 would only consider tags that match a given regular expression for update. In