[css-shadow-parts-1] Add missing "not" w3c#2412

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -350,7 +350,7 @@ The rules for parsing a part mapping are as follows:
     and let <var>second token</var> be the result.
 1. If <var>second token</var> is empty then return error.
 1. [=Collect a sequence of code points=] that are space characters.
-1. If <var>position</var> is past the end of <var>input</var> then return error.
+1. If <var>position</var> is not past the end of <var>input</var> then return error.
 1. Return the [=pair=] <var>first token</var>/<var>second token</var>.
 
 Rules for parsing a list of part mappings {#parsing-mapping-list}