Use square brackets in Tuples examples

[dfuenzalida]

Two of the Struct examples at https://janet-lang.org/docs/syntax.html#Structs-1 won't work on the REPL:

$ janet
Janet 1.38.0-ddc12295 linux/x64/gcc - '(doc)' for help
repl:1:> {(1 2 3) (4 5 6)}
repl:1:2: compile error: 1 expects 1 argument, got 2
repl:2:> {'(1 2 3) '(4 5 6)}
{(1 2 3) (4 5 6)}

This PR changes 2 examples to the tuple literal (square brackets) that are introduced in the Tuples section right before Structs. For newcomers it's probably simpler to use the square brackets rather than using quote (') at this point in the docs (quote is introduced in the Special Forms section, right after Structs).

[sogaiu]

LGTM

[pepe]

LGTM