docs(pillar.example): fix TOFS comment to explain the default path [skip ci]

* Replace the Jinja brackets to avoid evaluating when `pillar.example` is
  used from Kitchen
* References:
  - https://github.com/saltstack-formulas/libvirt-formula/pull/60#issuecomment-537965254
  - https://github.com/saltstack-formulas/libvirt-formula/pull/60#issuecomment-537988138

pillar.example

     #
     # All aspects of path/file resolution are customisable using the options below.
     # This is unnecessary in most cases; there are sensible defaults.
-    # Default path: salt://{{ path_prefix }}/{{ dirs.files }}/{{ dirs.default }}
+    # Default path: salt://< path_prefix >/< dirs.files >/< dirs.default >
     #         I.e.: salt://nginx/files/default
     # path_prefix: template_alt
     # dirs: