man-pages: Fix incorrect command line argument in documentation

[gal-pressman]

The --with_flexible_array and --with_embedded_flexible_array arguments were wrongly documented with the short argument '-P' (copy-paste mistake from previous argument), remove it.

Fixes: 7eea706c1499 ("pahole: Introduce --with_flexible_array option to show just types ending in a flexible array") Fixes: 446c28d118b9 ("pahole: Introduce --with_embedded_flexible_array")