[SF.9] singular/plural mismatch (closes #1545)

CppCoreGuidelines.md

@@ -18988,8 +18988,9 @@ Our recommendation is to write in ISO C++: See [rule P.2](#Rp-Cplusplus).
 
 ##### Reason
 
-Cycles complicates comprehension and slows down compilation.
-Complicates conversion to use language-supported modules (when they become available).
+Cycles complicate comprehension and slow down compilation. They also
+complicate conversion to use language-supported modules (when they become
+available).
 
 ##### Note