[basic.start.main] fix definite article (#7121)

Author: Andreas-Krug

source/basic.tex

@@ -6784,7 +6784,7 @@
 \tcode{argv[0]} through \tcode{argv[argc-1]} as pointers to the initial
 characters of null-terminated multibyte strings (\ntmbs{}s)\iref{multibyte.strings}
 and \tcode{argv[0]} shall be the pointer to
-the initial character of a \ntmbs{} that represents the name used to
+the initial character of an \ntmbs{} that represents the name used to
 invoke the program or \tcode{""}. The value of \tcode{argc} shall be
 non-negative. The value of \tcode{argv[argc]} shall be 0.