Fix publishdate date error / typo in backstage-plugin.md

[yada]

Fix front matter date parsing error in backstage-plugin.md

Description:
The backstage-plugin.md guide currently triggers the following error during site generation (using hugo):

ERROR the "publishdate" front matter field is not a parsable date: backstage-plugin.md

Fix the invalid publishdate value to ensure proper parsing.

before:
publishdate: 205-09-29

after:
publishdate: 2025-09-29