Update tablet-3-variables.txt

Link fix:
[are listed their
table](ap-b-lookup.html#special-variables). To understand
their secondary sigil [go to the twigil chapter of this
tablet](variables.html#twigils).

*** becomes ***

[are listed their
table](ap-b-lookup.html#special-variables). To understand
their secondary sigil [go to the twigil chapter of this
tablet](tablet-3-variables.html#twigils).