Fix type

[hannut91]

Fix typo of scss file. Replace from '#' to =.

Error message:

Conversion error: Jekyll::Converters::Scss encountered an error while converting 'assets/css/i.scss':
Error: Invalid CSS after "...t(startColorstr": expected expression (e.g. 1px, bold), was "#$color,endColorstr" on line 87:77 of _sass/_mixins.scss from line 8:1 of i.scss >> Transform.Microsoft.gradient(startColorstr#$color,endColorstr=$color); 
------------------------------------------^
------------------------------------------------
Jekyll 4.0.0   Please append `--trace` to the `serve` command for any additional 
information or backtrace.
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[bartn]

please merge this pull request.