fix minor spelling mistake

Author: jtbx

std/exception.d

@@ -1542,7 +1542,7 @@ version (StdUnittest)
 /+
 Returns true if the field at index `i` in $(D T) shares its address with another field.
 
-Note: This does not merelly check if the field is a member of an union, but also that
+Note: This does not merely check if the field is a member of an union, but also that
 it is not a single child.
 +/
 package enum isUnionAliased(T, size_t i) = isUnionAliasedImpl!T(T.tupleof[i].offsetof);