Use PARTY_SIZE for Pokerus Nurse Conversation

src/field_specials.c

@@ -1412,7 +1412,7 @@ bool8 ScriptCheckFreePokemonStorageSpace(void)
 
 bool8 IsPokerusInParty(void)
 {
-    if (!CheckPartyPokerus(gPlayerParty, 0x3f))
+    if (!CheckPartyPokerus(gPlayerParty, (1 << PARTY_SIZE) - 1))
         return FALSE;
 
     return TRUE;