Fix URL in submodule API example

What does this MR do?

Fixes the example URL in the submodule API documentation to match the endpoint introduced in https://gitlab.com/gitlab-org/gitlab-ce/blob/28cbb2acfe413148ff23b8ed4b3293e09ab376f5/spec/requests/api/submodules_spec.rb#L28.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards