[value.error.codes] Fix indefinite article (#8730)

Author: Andreas-Krug

source/lib-intro.tex

@@ -3977,7 +3977,7 @@
 \rSec3[value.error.codes]{Value of error codes}
 
 \pnum
-Certain functions in the \Cpp{} standard library report errors via a
+Certain functions in the \Cpp{} standard library report errors via an
 \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