Update doc/how-to-package-haskell-code.rst

Co-authored-by: brandon s allbery kf8nh <allbery.b@gmail.com>

doc/how-to-package-haskell-code.rst

@@ -104,7 +104,7 @@ can build and run our application.
    ...
    Hello, Haskell!
 
-Since we have an executable we can use ``cabal run proglet`` which will build
+Since we have an executable, we can use ``cabal run proglet`` which will build
 our executable (and re-build it if we've made any changes) and then run the
 binary. The ``cabal run`` command works for any ``component-name`` (tests for
 example), not just the main executable.