[defns.impl.limits] Use singular

Fixes ISO/CS 015 (C++23 DIS).

Author: tkoeppe

source/intro.tex

@@ -337,8 +337,8 @@
 \tcode{<}, \tcode{>}, \tcode{<=}, \tcode{>=}, and \tcode{<=>}
 
 \indexdefn{limits!implementation}%
-\definition{implementation limits}{defns.impl.limits}
-restrictions imposed upon programs by the implementation
+\definition{implementation limit}{defns.impl.limits}
+restriction imposed upon programs by the implementation
 
 \indexdefn{behavior!locale-specific}%
 \definition{locale-specific behavior}{defns.locale.specific}