[typed-om] Use the correct name from WebIDL for indexed property getter.

Author: tabatkins

css-typed-om/Overview.bs

@@ -2216,7 +2216,7 @@ of all the "math" operations.
 
 The <dfn attribute for=CSSNumericArray>length</dfn> attribute of {{CSSNumericArray}} indicates how many {{CSSNumericValue}}s are contained within the {{CSSNumericArray}}.
 
-The <dfn export for=CSSNumericArray>indexed getter</dfn> of {{CSSNumericArray}} retrieves the {{CSSNumericValue}} at the provided index.
+The <dfn export for=CSSNumericArray>[=/indexed property getter=]</dfn> of {{CSSNumericArray}} retrieves the {{CSSNumericValue}} at the provided index.
 
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