[stdatomic.h.syn] Use "declare" for typedef-names

Author: frederick-vs-ja

source/threads.tex

@@ -6121,7 +6121,7 @@
 \pnum
 Each of the \grammarterm{using-declaration}s for
 \tcode{int$N$_t}, \tcode{uint$N$_t}, \tcode{intptr_t}, and \tcode{uintptr_t}
-listed above is defined if and only if the implementation defines
+listed above is declared if and only if the implementation declares
 the corresponding \grammarterm{typedef-name} in \ref{atomics.syn}.
 
 \pnum