doc: Update the File Systems samples link syntax

The commit 16183f66 ("samples: subsys: fs: create common fs sample")
renames the sample name for FAT fs from 'fat_fs' to 'fs_sample'.

To avoid this kind of errors, ":zephyr:code-sample:`xxx`" is introduced.

Signed-off-by: Haiyue Wang <haiyuewa@163.com>