Fix markdown spoiler

[NiKuma0]

Description

In telegram bot api documentation said that need two characters, but there we have only one. I did catch a bug with this, but github issue I didn't find and don't want to create.

Type of change

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

How Has This Been Tested?

I tested it on my own project, but you can use this code:

...
@router.message()
async def test(m: types.Message):
    return await m.answer(m.md_text)

[github-actions]

✔️ Changelog found.

Thank you for adding a description of the changes

[JrooTJunior]

Thanks for you contribution!

Please add short explanation of the changes in CHANGES/1176.bugfix.rst file and update tests

[JrooTJunior]

... and update tests

I see that tests still not updated

Just look at test results: https://github.com/aiogram/aiogram/actions/runs/4951994811/jobs/8858509304?pr=1176

[codecov]

Codecov Report

Merging #1176 (c8dd635) into dev-3.x (cf269e1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           dev-3.x     #1176   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          358       358           
  Lines         8841      8841           
=========================================
  Hits          8841      8841           

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/text_decorations.py	100.00% <100.00%> (ø)