doc: Update the File Systems samples link syntax

The commit 16183f665e ("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>

doc/services/file_system/index.rst

@@ -51,10 +51,10 @@ Samples for the VFS are mainly supplied in ``samples/subsys/fs``, although vario
 VFS usage are provided as important functionalities in samples for different subsystems.
 Here is the list of samples worth looking at:
 
-- ``samples/subsys/fs/fat_fs`` is an example of FAT file system usage with SDHC media;
-- ``samples/subsys/shell/fs`` is an example of Shell fs subsystem, using internal flash partition
+- :zephyr:code-sample:`fs` is an example of FAT file system usage with SDHC media;
+- :zephyr:code-sample:`shell-fs` is an example of Shell fs subsystem, using internal flash partition
   formatted to LittleFS;
-- ``samples/subsys/usb/mass/`` example of USB Mass Storage device that uses FAT FS driver with RAM
+- :zephyr:code-sample:`usb-mass` example of USB Mass Storage device that uses FAT FS driver with RAM
   or SPI connected FLASH, or LittleFS in flash, depending on the sample configuration.
 
 API Reference