[value.error.codes] Remove redundant std qualifications (#7918)

Author: jwakely

source/lib-intro.tex

@@ -3966,8 +3966,8 @@
 
 \pnum
 Certain functions in the \Cpp{} standard library report errors via a
-\tcode{std::error_code}\iref{syserr.errcode.overview} object. That object's
-\tcode{category()} member shall return \tcode{std::system_category()} for
+\tcode{error_code}\iref{syserr.errcode.overview} object. That object's
+\tcode{category()} member shall return \tcode{system_category()} for
 errors originating from the operating system, or a reference to an
 \impldef{\tcode{error_category} for errors originating outside the
 operating system} \tcode{error_category} object for errors originating elsewhere.