Fix package-lint/byte-compiler/check-doc warnings

[conao3]

Hi!
I find this awesome package and there are warnings from some
linters.
So I fix them for my first contribution step!

before

 ox-gfm.el    89     info            First line is not a complete sentence (emacs-lisp-checkdoc)
 ox-gfm.el    90     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   110   1 error           "width-cookies" doesn't start with package's prefix "ox-gfm". (emacs-lisp-package)
 ox-gfm.el   111   1 error           "width-cookies-table" doesn't start with package's prefix "ox-gfm". (emacs-lisp-package)
 ox-gfm.el   113   1 error           "gfm-table-left-border" doesn't start with package's prefix "ox-gfm". (emacs-lisp-package)
 ox-gfm.el   114   1 error           "gfm-table-right-border" doesn't start with package's prefix "ox-gfm". (emacs-lisp-package)
 ox-gfm.el   115   1 error           "gfm-table-separator" doesn't start with package's prefix "ox-gfm". (emacs-lisp-package)
 ox-gfm.el   118     info            First line is not a complete sentence (emacs-lisp-checkdoc)
 ox-gfm.el   118     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   119     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   152     info            First line is not a complete sentence (emacs-lisp-checkdoc)
 ox-gfm.el   153     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   164     info            First line is not a complete sentence (emacs-lisp-checkdoc)
 ox-gfm.el   164     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   165     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   182     info            First line is not a complete sentence (emacs-lisp-checkdoc)
 ox-gfm.el   182     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   183     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   209     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   236     info            First line is not a complete sentence (emacs-lisp-checkdoc)
 ox-gfm.el   236     info            There should be two spaces after a period (emacs-lisp-checkdoc)
 ox-gfm.el   239  53 warning         reference to free variable ‘info’ (emacs-lisp)
 ox-gfm.el   327     info            First line is not a complete sentence (emacs-lisp-checkdoc)
 ox-gfm.el   327     info            Lisp symbol ‘org-mode’ should appear in quotes (emacs-lisp-checkdoc)

after

 ox-gfm.el   240  53 warning         reference to free variable ‘info’ (emacs-lisp)

NOTE:
I think remained warning is fatal.
Where info from? Missing function argument specification or other?