[format.arg] Fix indefinite article (#7536)

Author: Andreas-Krug

source/text.tex

@@ -8492,7 +8492,7 @@
 \pnum
 \expects
 If \tcode{decay_t<T>} is \tcode{char_type*} or \tcode{const char_type*},
-\tcode{static_cast<const char_\linebreak{}type*>(v)} points to a NTCTS\iref{defns.ntcts}.
+\tcode{static_cast<const char_\linebreak{}type*>(v)} points to an NTCTS\iref{defns.ntcts}.
 
 \pnum
 \effects