Fix remove member from role

[minchao]

Related issue

#74

Proposed changes

The member to be removed should be a string in the path.

keto/engine/ladon/doc.go

Lines 162 to 163 in fc1ced6

	// in: body
	Body removeOryAccessControlPolicyRoleMembersBody

Checklist

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• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

[aeneasr]

Thank you!