Fix syntax for rubocop

[jhunschejones]

Description:

@thomasfl / @AlexWayfer CI started failing in September for a rubocop formatting offense:
From Cirrus Logs

bundle exec rubocop
Inspecting 15 files
........W......
Offenses:
lib/filewatcher/spec_helper/watch_run.rb:16:16: W: [Correctable] Lint/ElseLayout: Odd else layout detected. Did you mean to use elsif?
          else File.join(TMP_DIR, filename)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
15 files inspected, 1 offense detected, 1 offense auto-correctable

This PR just makes a small tweak to the formatting of the line in question to make rubocop happy again:

bundle exec rubocop
Inspecting 15 files
...............

15 files inspected, no offenses detected

Testing:

All tests should continue to pass

[AlexWayfer]

Thank you for the patch!

But… there are 2 moments:

1. I'd like to enforce RuboCop version lock to the patch version for receiving new cops (enabled by our config) only in Depfu-bot PRs (or similar) with separated CI runs.
2. An easy search forwards me to the RuboCop's issue: Fix Lint/ElseLayout when using multi-branch statements rubocop/rubocop#10147
   And I see a kind of consensus there that this (Lint/*Layout, LOL) cop should check consistency of if-else blocks styles (and account if-then + one-line else) instead of just simple "ouh, there is one-line else, this is wrong!".

I like this code style, and until RuboCop (surely) forces to another (multi-line) — I wouldn't like to change.

(BTW you can create a small PR for the first point with a lock to the latest successful version, like 1.21, as I see)
(never mind: I've already pushed this to the master 30b41b7 while inspecting additional details and discussions)

[jhunschejones]

Awesome, sounds like the build is fixed then! I'll close out this PR.