Fixes typo in #discovery_permissions= method name

Introduced in commit 71ff36206e8ce08175de87b2a0b96620f49a5221.

I get one CI test failure, but not related to this change:

```
  1) Hydra::PolicyAwareAccessControlsEnforcement apply_gated_discovery should include policy-aware query
     Failure/Error: @solr_parameters[:fq].first.should include(" OR (_query_:\"{!raw f=#{governed_field}}info:fedora/test:policy1\" OR _query_:\"{!raw f=#{governed_field}}info:fedora/test:policy2\" OR _query_:\"{!raw f=#{governed_field}}info:fedora/test:policy3\" OR _query_:\"{!raw f=#{governed_field}}info:fedora/test:policy4\" OR _query_:\"{!raw f=#{governed_field}}info:fedora/test:policy5\" OR _query_:\"{!raw f=#{governed_field}}info:fedora/test:policy6\" OR _query_:\"{!raw f=#{governed_field}}info:fedora/test:policy7\" OR _query_:\"{!raw f=#{governed_field}}info:fedora/test:policy8\")")
       expected "edit_access_group_ssim:public OR discover_access_group_ssim:public OR read_access_group_ssim:public OR edit_access_group_ssim:public OR discover_access_group_ssim:public OR read_access_group_ssim:public OR edit_access_group_ssim:student OR discover_access_group_ssim:student OR read_access_group_ssim:student OR edit_access_group_ssim:africana-104-students OR discover_access_group_ssim:africana-104-students OR read_access_group_ssim:africana-104-students OR edit_access_group_ssim:registered OR discover_access_group_ssim:registered OR read_access_group_ssim:registered OR edit_access_person_ssim:sara_student OR discover_access_person_ssim:sara_student OR read_access_person_ssim:sara_student OR (is_governed_by_ssim:info\\:fedora\\/test\\:policy1 OR is_governed_by_ssim:info\\:fedora\\/test\\:policy2 OR is_governed_by_ssim:info\\:fedora\\/test\\:policy3 OR is_governed_by_ssim:info\\:fedora\\/test\\:policy4 OR is_governed_by_ssim:info\\:fedora\\/test\\:policy5 OR is_governed_by_ssim:info\\:fedora\\/test\\:policy6 OR is_governed_by_ssim:info\\:fedora\\/test\\:policy7 OR is_governed_by_ssim:info\\:fedora\\/test\\:policy8)" to include " OR (_query_:\"{!raw f=is_governed_by_ssim}info:fedora/test:policy1\" OR _query_:\"{!raw f=is_governed_by_ssim}info:fedora/test:policy2\" OR _query_:\"{!raw f=is_governed_by_ssim}info:fedora/test:policy3\" OR _query_:\"{!raw f=is_governed_by_ssim}info:fedora/test:policy4\" OR _query_:\"{!raw f=is_governed_by_ssim}info:fedora/test:policy5\" OR _query_:\"{!raw f=is_governed_by_ssim}info:fedora/test:policy6\" OR _query_:\"{!raw f=is_governed_by_ssim}info:fedora/test:policy7\" OR _query_:\"{!raw f=is_governed_by_ssim}info:fedora/test:policy8\")"
     # ./spec/unit/policy_aware_access_controls_enforcement_spec.rb:124:in `block (3 levels) in <top (required)>'
```

This same test is passing in Travis, so must be something in my Ruby environment?