`TypeAlias` syntax missing a `TypeParamBounds`

[GilRtr]

this is valid syntactically:

type TypeAlias: TraitBound = SomeType;

however it is not part of the reference.

expectation:

^Syntax
TypeAlias :
   type IDENTIFIER GenericParams^? ( : TypeParamBounds )^? WhereClause^? ( = Type )^? ;

*should be stated that this syntax is only useful in the context of an associated type and will yield bounds on `type`s in this context have no effect anywhere else.

edit: I accidently pressed enter before writing the description

[ehuss]

Thanks for the eagle eye! Looks like I missed this as part of the grand parser unification (#927). Would you be willing to open a PR?