Correct substitution for bulgarian 'Щ'

[kvalev]

Probably due to a copy-paste error the substitution for Ш and Щ was identical, but only for the uppercase variants.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (76470d7) to head (d923cc0).

:exclamation: Current head d923cc0 differs from pull request most recent head b33dd82. Consider uploading reports for the commit b33dd82 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #80 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 2 2 Lines 119 142 +23 ========================================= + Hits 119 142 +23 ``` | [Files](https://app.codecov.io/gh/gosimple/slug/pull/80?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gosimple) | Coverage Δ | | |---|---|---| | [languages\_substitution.go](https://app.codecov.io/gh/gosimple/slug/pull/80?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gosimple#diff-bGFuZ3VhZ2VzX3N1YnN0aXR1dGlvbi5nbw==) | `100.00% <ø> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/gosimple/slug/pull/80/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gosimple)

[kvalev]

@matrixik Can you take a look when you get the chance?

[matrixik]

Thank you for your fix and sorry it took so long.