Fix documentation xref

Antora currently fails with

    [11:39:31.695] ERROR (asciidoctor): target of xref not found: cops_rspec_rails.adoc
      file: docs/modules/ROOT/nav.adoc
      source: https://github.com/rubocop/rubocop-rspec_rails.git (tag: v2.28.0 | start path: docs)
    [11:39:31.742] ERROR (asciidoctor): target of xref not found: cops_rspec_rails.adoc
      file: docs/modules/ROOT/nav.adoc
      source: https://github.com/rubocop/rubocop-rspec_rails.git (tag: v2.29.0 | start path: docs)
    [11:39:31.756] ERROR (asciidoctor): target of xref not found: cops_rspec_rails.adoc
      file: docs/modules/ROOT/nav.adoc
      source: https://github.com/rubocop/rubocop-rspec_rails.git (branch: master | start path: docs)

Author: bquorning

docs/modules/ROOT/nav.adoc

@@ -4,4 +4,4 @@
 * xref:cops.adoc[Cops]
 * xref:development.adoc[Development]
 * Cops Documentation
-** xref:cops_rspec_rails.adoc[Rails]
+** xref:cops_rspecrails.adoc[Rails]