copilot-core: Fix typo in documentation. Refs #587.

This commit fixes a typo (a sentence ending in double period) in the
documentation of `copilot-core`.

Author: ivanperez-keera

copilot-core/src/Copilot/Core/Spec.hs

@@ -13,7 +13,7 @@
 --
 -- In order to be executed, high-level Copilot Language Spec must be turned
 -- into Copilot Core's 'Spec'. This module defines the low-level Copilot Core
--- representations for Specs and the main types of element in a spec..
+-- representations for Specs and the main types of element in a spec.
 module Copilot.Core.Spec
     ( Stream (..)
     , Observer (..)