fix: aspectRatio type in schema

[jpoehnelt]

Incorrect type. Expected "number".yaml-schema: 
file:///.../antfu.slidev-51.3.0/schema/headmatter.json
Aspect ratio for slides should be like 16/9 or 1:1

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[kermanx]

It seems that

slidev/packages/parser/src/config.ts

Line 95 in dd883b7

config.aspectRatio = parseAspectRatio(config.aspectRatio)

has normalized the value. All the additions of parseAspectRatio in this PR seem to be unnecessary. Correct me if I am wrong

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commit: a0904d7