Renaming picture fallback to img is not reflected in the document

[Kim, Harim]

Picutre's fallback property was renamed to img in 4.0.0

imagetools/packages/core/CHANGELOG.md

Line 45 in 22f6de6

 - 861276f: breaking: rename picture fallback to img 

But it doesn't reflected in the document, which confuses

imagetools/docs/interfaces/vite_src.Picture.md

Line 13 in 22f6de6

 - [fallback](vite_src.Picture.md#fallback) 

imagetools/docs/directives.md

Line 437 in 22f6de6

 html += `<img src={image.fallback.src} /></picture>` 

[Ben McCann]

fixed