doc: Recommend alist-copy instead of list-copy.

The current recommendation of `list-copy' is not right and does not lead
to preserving the original list:

    scheme@(guile-user)> (define x (list (cons 'a 1) (cons 'b 2)))
    scheme@(guile-user)> (define y (list-copy x))
    scheme@(guile-user)> (assq-set! y 'b 3)
    $1 = ((a . 1) (b . 3))
    scheme@(guile-user)> x
    $2 = ((a . 1) (b . 3))

Correct approach seems to be use `alist-copy' from SRFI-1 leading to the
expected behavior of:

    scheme@(guile-user)> ,use (srfi srfi-1)
    scheme@(guile-user)> (define x (list (cons 'a 1) (cons 'b 2)))
    scheme@(guile-user)> (define y (alist-copy x))
    scheme@(guile-user)> (assq-set! y 'b 3)
    $1 = ((a . 1) (b . 3))
    scheme@(guile-user)> x
    $2 = ((a . 1) (b . 2))

* doc/ref/api-data.texi (Adding or Setting Alist Entries): Recommend
`alist-copy'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/ref/api-data.texi

@@ -9510,7 +9510,7 @@ difference to you.
 If you need to keep the old value of an association list in a form
 independent from the list that results from modification by
 @code{acons}, @code{assq-set!}, @code{assv-set!} or @code{assoc-set!},
-use @code{list-copy} to copy the old association list before modifying
+use @code{alist-copy} to copy the old association list before modifying
 it.
 
 @rnindex acons