mirage/mirage: PR #1618: fix description in opam file

@25fafc2

Stages:

• ✔️ x86_64-debian-13-4.14 (3m43s)
• ✔️ arm64-debian-13-4.14 (3m58s)
• ✔️ x86_64-debian-13-5.3 (4m40s)
• ✔️ arm64-debian-13-5.3 (4m57s)

History:

• 09/10/2025 20:14 ✔️ @b582a7a (22m43s)
• 09/10/2025 08:33 ✔️ @25fafc2 (4m57s)
• 09/10/2025 07:42 ❌ @a42fc02 (3m14s)