[4.0] Fix missing $ for scss variable cassiopeia-grid-gutter

[hans2103]

Pull Request for Issue # .

Summary of Changes

This PR just adds a missing $ to a scss variable

Testing Instructions

• npm run build:css to compile the css
• check the css output
• apply PR
• npm run build:css to compile the css
• check the css output and compare

Actual result BEFORE applying this Pull Request

.site-grid .container-component > div + div,
.site-grid .container-sidebar-left > div + div,
.site-grid .container-sidebar-right > div + div {
   margin-top: cassiopeia-grid-gutter;
}

Expected result AFTER applying this Pull Request

.site-grid .container-component > div + div,
.site-grid .container-sidebar-left > div + div,
.site-grid .container-sidebar-right > div + div {
   margin-top: 15px;
}

Documentation Changes Required

No changes required

[Quy]

I have tested this item ✅ successfully on 1885f24

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/29856.}

[wilsonge]

Merged with the good test and on review