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

@a42fc02

Stages:

• ❌ x86_64-debian-13-4.14 (3m14s)
• ❌ arm64-debian-13-4.14 (2m16s)
• ❌ x86_64-debian-13-5.3 (2m22s)
• ❌ arm64-debian-13-5.3 (2m00s)

History:

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