Fix warnings for flower.el

[profitware]

Fix warnings for flower.el as stated by @riscy here: melpa/melpa#7991 (comment)

flower.el with byte-compile using Emacs 27.1:

flower.el:226:2:Warning: `org-add-link-type' is an obsolete function (as of
    Org 9.0); use `org-link-set-parameters' instead.

flower.el with package-lint 20220412.1648:

1 issue found:
1:0: warning: The package summary should not end with a period.