PG-kura

[stzn/the-swift-programming-language-jp] Issue opened: #490 A typo exists in the Properties page

The leading s of struct is missing. https://github.com/stzn/the-swift-programming-language-jp/blob/495a09a28f08d3f0c0168bae3169b65fb757c5b7/language-guide/properties.md?plain=1#L389