fix API mistake
fix API mistake

In _The Model Factory_ section, modelFactory.parse expects `TemplateData` instead of `String`

`context.getTemplateData().getTemplate()` => `context.getTemplateData()`
thymeleaf/thymeleaf-docs,thymeleaf-docs-ja/thymeleaf-docs,thymeleaf-docs-ja/thymeleaf-docs,thymeleaf/thymeleaf-docs