fix '-s' help message in 'ubuild' command

Author: t-yamamura

README.md

@@ -296,7 +296,7 @@ optional arguments:
   -h, --help  show this help message and exit
   -d string   description comment to be embedded on dictionary
   -o file     output file (default: user.dic)
-  -s file     system dictionary (default: core dic)
+  -s file     system dictionary path (default: system core dictionary path)
 ```
 
 About the dictionary file format, please refer to [this document](https://github.com/WorksApplications/Sudachi/blob/develop/docs/user_dict.md) (written in Japanese, English version is not available yet).

docs/tutorial.md

@@ -293,7 +293,7 @@ optional arguments:
   -h, --help  show this help message and exit
   -d string   description comment to be embedded on dictionary
   -o file     output file (default: user.dic)
-  -s file     system dictionary (default: core dic)
+  -s file     system dictionary path (default: system core dictionary path)
 ```
 
 辞書ファイル形式については[user_dict.md](https://github.com/WorksApplications/Sudachi/blob/develop/docs/user_dict.md)を参照してください。 

sudachipy/command_line.py

@@ -207,7 +207,7 @@ def main():
     parser_ubd.add_argument('-o', dest='out_file', metavar='file', default='user.dic',
                             help='output file (default: user.dic)')
     parser_ubd.add_argument('-s', dest='system_dic', metavar='file', required=False,
-                            help='system dictionary (default: core dic)')
+                            help='system dictionary path (default: system core dictionary path)')
     parser_ubd.add_argument("in_files", metavar="file", nargs=argparse.ONE_OR_MORE,
                             help='source files with CSV format (one or more)')
     parser_ubd.set_defaults(handler=_command_user_build, print_usage=parser_ubd.print_usage)