pathd: fix excessive doc string on pcep msd command

pathd: fix excessive doc string on pcep msd command

The following message appears after starting pathd:
> root@ubuntu2204:~# vtysh
> 2023/10/31 05:59:10 [K2CCG-5Y7ZJ] Excessive docstring while parsing 'msd (1-32)'
> 2023/10/31 05:59:10 [W7ENN-K2SVA] ----------
> 2023/10/31 05:59:10 [WCW75-6TZPF] PCC maximum SID depth value
> 2023/10/31 05:59:10 [W7ENN-K2SVA] ----------

Remove the excessive doc string in the pcep msd command.

Fixes: d6d2527448ef ("pathd: fix 'no msd' command not possible under pcc node")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>