[mkdocs] Missing backslashes in test/plot-schema.json

In test/plot-schema.json, this sentence: The URI consists of \"data:image/[<media subtype>][;base64],<data>\"" does not render properly because the brackets are not escaped. This happens when test/plot-schema.json is copied over into plotly.py docs and used to render docstrings (and eventually into markdown). If you add the necessary backslashes, then it will fix the rendering in plotly.py docs but render with the backslashes in plotly.js.

从 test/plot-schema.json 中的句子开始，跟踪该文件如何被复制到 plotly.py 文档中，然后在 markdown 或 plotly.js 中渲染。比较两条路径中的输出；只有在括号能够正确渲染且不会在 plotly.js 中引入不需要的反斜杠时，issue 才算完成。