Fix SASS syntax errors from #967

This introduced syntax errors that were not picked up in review, the
`:not()` psuedo selector can't lead a selector in SASS.

Author: agjohnson

sphinx_rtd_theme/static/css/theme.css

@@ -515,10 +515,10 @@
     border-radius: $base-line-height / 6
     padding: ($base-line-height / 10) ($base-line-height / 4)
     margin: auto ($base-line-height / 12)
-  :not(kbd) > kbd,
-  :not(kbd) > .kbd,
-  :not(.kbd) > kbd,
-  :not(.kbd) > .kbd
+  &:not(kbd) > kbd,
+  &:not(kbd) > .kbd,
+  &:not(.kbd) > kbd,
+  &:not(.kbd) > .kbd
     color: inherit
     font-size: 80%
     background-color: lighten($text-light, 30%)