(maint) - Update fixtures/modules path for .gitignore

Prior to this commit, the path supplied to .gitignore for
spec/fixtures/modules/ did not allow for manual unsetting of certain
file paths and therefore the check:gitignore check for linter threw an
error. "File specified in .gitignore has been committed"

To give more context, take the puppetlabs-node_encrypt module. The
node_encrypt module defines a module called "redact" within
spec/fixtures/modules/redact/, and this is required for spec testing.
However, once the `bundle exec rake syntax lint check:git_ignore`
command is run, we get the error seen above.

This commit alters the required path added to the .gitignore of the
puppet modules, but does not alter its core behaviour. (it will still
ignore all files under /spec/fixtures/modules), however now we can pass
additional paths to exclude from .gitignore (i.e.
spec/fixtures/modules/redact/) in our .sync.yml.