Update a_swift_tour.md

“In an if statement, the conditional must be a Boolean expression—this means that code such as if score { ... } is an error, not an implicit comparison to zero.”  译文中 “not an implicit comparison to zero”未做翻译。
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Add content of Naming constants and Variables
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初步完成 Using Swift with Cocoa and Objective-C 第三章
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complete Naming Constants and Printing Constants

complete Naming Constants and Variables and Printing Constants and
Variables
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Strings and Characters 02/03
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complete Naming Constants and Printing Constants
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