Auto merge of #11964 - ConnorGBrewster:fix_ua_comment, r=asajeffrey

Use proper comment syntax in servo.css

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11963 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because using proper comment syntax in css.

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resources/servo.css

@@ -132,7 +132,7 @@ textarea {
   -servo-overflow-clip-box: content-box;
 }
 
-// https://html.spec.whatwg.org/multipage/rendering.html#the-details-and-summary-elements
+/* https://html.spec.whatwg.org/multipage/rendering.html#the-details-and-summary-elements */
 details {
   display: block;
 }