Fix a link to docs for Mask CI/CD variable from Settings

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

app/views/ci/variables/_index.html.haml

@@ -17,7 +17,7 @@
   aws_tip_commands_link: help_page_path('ci/cloud_deployment/index.md', anchor: 'run-aws-commands-from-gitlab-cicd'),
   aws_tip_learn_link: help_page_path('ci/cloud_deployment/index.md', anchor: 'aws'),
   protected_environment_variables_link: help_page_path('ci/variables/README', anchor: 'protect-a-cicd-variable'),
-  masked_environment_variables_link: help_page_path('ci/variables/README', anchor: 'mask-a-custom-variable'),
+  masked_environment_variables_link: help_page_path('ci/variables/README', anchor: 'mask-a-cicd-variable'),
 } }
 
 - if !@group && @project.group