Fix navbar links

It was referring to an anchor on index instead of a different page:
https://docs.rubocop.org/rubocop-rspec/index.html#third_party_rspec_extensions.adoc

Author: pirj

docs/modules/ROOT/nav.adoc

@@ -3,9 +3,9 @@
 * xref:usage.adoc[Usage]
 * xref:cops.adoc[Cops]
 * xref:upgrade_to_version_2.adoc[Upgrade to 2.x]
-* xref:third_party_rspec_extensions.adoc[RSpec syntax extensions in third-party gems]
+* xref:third_party_rspec_syntax_extensions.adoc[RSpec syntax extensions in third-party gems]
 * Cops Documentation
-** xref:cops_capybara.adoc[Capybara]
-** xref:cops_factorybot.adoc[FactoryBot]
-** xref:cops_rails.adoc[Rails]
+** xref:cops_rspec/capybara.adoc[Capybara]
+** xref:cops_rspec/factorybot.adoc[FactoryBot]
+** xref:cops_rspec/rails.adoc[Rails]
 ** xref:cops_rspec.adoc[RSpec]