fixing some call_modify examples w/ NULL -> zap() #1767
                
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Some examples in call_modify() predate the introduction of zap() and use the older
[arg] = NULLsyntax. In particular, this made line 560's example,call_modify(call, bar = NULL, bar = missing_arg()), no longer illustrate its key behavior. Changed that, plus 2 other (likely less important) examples where argument removal seems to have been intended.