Prevent hh-apidoc from being upgraded to an incompatible version

[Atry]

According to https://github.com/hhvm/hh-apidoc/releases/tag/v0.8.0 , hh-apidoc is not backward compatible with 0.6.0.

[fredemmott]

Change is good, however:

This PR should fix the nightly CI

There is a lock file; this should be an unobservable change by itself. It should have no effect on CI, and the failure appears unrelated.

[fredemmott]

FAILED: build/extracted-examples/api/hack/class.AsyncMysqlQueryResult.method.vectorRowsTyped/basic-usage.hack
[10](https://github.com/hhvm/user-documentation/runs/5186482140?check_suite_focus=true#step:6:10)
invalid skipif output 'skipterminate called after throwing an instance of 'std::system_error'
[11](https://github.com/hhvm/user-documentation/runs/5186482140?check_suite_focus=true#step:6:11)
  what():  pthread_setspecific failed: Invalid argument'

This is an intermittent environment issue or HHVM bug, not a Hack library compatibility one.