docs: Fix typo

[sebazelonka]

When I use the command opencode agent create the agent.md file is created under agent folder instead of agents so I updated the docs to match the current behavior.

Updated paths for agent markdown files from 'agents' to 'agent'.

What does this PR do?

Fix a typo I found

[github-actions]

Hey! Your PR title Fix typo doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[rekram1-node]

We actually want all the docs to mention the plurals since so many people complain, can you update agents command instead to match docs?