reference doc: fix incorrect syntax production

[SethTisue]

as reported by @bjornregnell on Discord

this brings the doc page in line with https://github.com/lampepfl/dotty/blob/main/docs/_docs/internals/syntax.md

[bjornregnell]

related to #19016

[som-snytt]

This is one of those weird dotty does it this way things?

➜  dotty git:(issue/11723-doc-param-order) grep StructuralIn $(find docs -type f)
docs/_docs/internals/syntax.md:GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
docs/_docs/internals/syntax.md:StructuralInstance ::=  ConstrApp {‘with’ ConstrApp} [‘with’ WithTemplateBody]
docs/_docs/reference/contextual/givens.md:GivenDef            ::=  [GivenSig] StructuralInstance
docs/_docs/reference/contextual/givens.md:StructuralInstance  ::=  ConstrApp {‘with’ ConstrApp} ‘with’ TemplateBody
docs/_docs/reference/syntax.md:GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
docs/_docs/reference/syntax.md:StructuralInstance ::=  ConstrApp {‘with’ ConstrApp} [‘with’ WithTemplateBody]
docs/_spec/TODOreference/contextual/givens.md:GivenDef            ::=  [GivenSig] StructuralInstance
docs/_spec/TODOreference/contextual/givens.md:StructuralInstance  ::=  ConstrApp {‘with’ ConstrApp} ‘with’ TemplateBody
docs/_spec/13-syntax-summary.md:GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
docs/_spec/13-syntax-summary.md:StructuralInstance ::=  ConstrApp {‘with’ ConstrApp} [‘with’ WithTemplateBody]

Do I see one missing bracket under the TODO. Why am I scanning multiple bits of text by eye?

This is what computers are for. I mean computers before AI.

After AI, grep is like, maybe this one?

[som-snytt]

Maybe missed one?

My favorite ad is the guy gets off the elevator (lift) and dances to call me maybe. He is chewing gum, so I guess the import of the ad is all the things you can do while chewing gum. It's quite amazing, and I will try it.

[odersky]

Thanks for the fix!

[SethTisue]

Maybe missed one?

@som-snytt ah, thanks for calling this to our attention

@sjrd the TODOreference directory was added by Quentin in 34b836b when he was working on the spec. You're the specmeister these days; would a separate PR removing that directory in its entirety be accepted? (I'm hoping for a "yes", of course. If the files are useful, perhaps they could live on a branch?)

Regardless, merging this PR in its current form.

the TODOreference directory is a maintenance burden; can we remove it?