Built-in function 'tuple' is implemented in Sprig as 'list'.
(see https://github.com/Masterminds/sprig/blob/2625cd487a689ca112e1301a58b89b347ba2c994/functions.go#L237 )

Replace documentation references to 'tuple' with 'list' so there
is no need to explain what a tuple is.
Signed-off-by: Daniel Badea <daniel.badea@windriver.com>