Updater: Add missing directory slash for merge path

[ocean90]

Description Fixes #225.

How has this been tested?

Screenshots If applicable, add screenshots to show the visual change.

Types of changes

Checklist:

• [ ] My code is tested.
• [ ] My code follows the WordPress code style.
• [ ] My code has proper inline documentation.

[codecov[bot]]

Codecov Report

Merging #226 (6333a6f) into master (0ec84b3) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #226   +/-   ##
=========================================
  Coverage     81.21%   81.21%           
  Complexity      369      369           
=========================================
  Files            24       24           
  Lines           942      942           
=========================================
  Hits            765      765           
  Misses          177      177           

Flag	Coverage Δ
feature	?
php	81.21% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
inc/Updater.php	94.11% <100.00%> (ø)

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