[easy] Fix type error in generics docs
[easy] Fix type error in generics docs

Summary: Closes https://github.com/facebook/flow/issues/7114

Reviewed By: jbrown215

Differential Revision: D12945793

fbshipit-source-id: ff34f94fb528ca80f0e720c25b7701e01e4729b0

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow