Fix escaping for markdown

[venticello]

Description

Fix escaping forbidden characters for markdown. The wrong decorator was used.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)

[github-actions]

❌ Changelog is required!

You need to add a brief description of the changes to the CHANGES directory.

For example, you can run towncrier create <issue>.<type> to create a file in the change directory and then write a description on that file.

Read more at Towncrier docs

[codecov]

Codecov Report

Merging #903 (9586bad) into dev-3.x (f2e02e2) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           dev-3.x      #903   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          300       300           
  Lines         8172      8172           
=========================================
  Hits          8172      8172           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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

Impacted Files	Coverage Δ
aiogram/utils/markdown.py	100.00% <100.00%> (ø)

[JrooTJunior]

Please add test that prove your fix and prevent to break this feature in the future

Also the short description of changes should be added as described above.

[JrooTJunior]

@venticello, any news?

[JrooTJunior]

Fixed in dev-3.x directly in due to author of this PR did not respond in too long time