fix tiny typo

Author: greg0ire

contributing/code/conventions.rst

@@ -75,4 +75,4 @@ must be used instead (where ``XXX`` is the name of the related thing):
     While "setXXX" and "replaceXXX" are very similar, there is one notable 
     difference: "setXXX" may replace, or add new elements to the relation. 
     "replaceXXX" on the other hand is specifically forbidden to add new 
-    elements, but most throw an exception in these cases.
+    elements, but must throw an exception in these cases.