Replace erroneous use of overflow

Modulo behaviour of unsigned integers appears to be referred to as overflow. However, [unsigned integers don't overflow](https://eel.is/c++draft/basic.fundamental#note-1). Replaced "overflow behavior" with "modulo behavior".

Author: johnmcfarlane

CppCoreGuidelines.md

@@ -13578,7 +13578,7 @@ but if you had seen `us - (s + 2)` or `s += 2; ...; us - s`, would you reliably
 ##### Exception
 
 Use unsigned types if you really want modular arithmetic - add
-comments as necessary noting the reliance on overflow behavior, as such code
+comments as necessary noting the reliance on modulo behavior, as such code
 is going to be surprising for many programmers.
 
 ##### Example