selftests/futex: Fix typos and grammar in futex_priv_hash

Fix multiple typos and small grammar issues in help text, comments and test
messages in the futex_priv_hash test.

Signed-off-by: Gopi Krishna Menon <krishnagopi487@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/20250827130011.677600-5-bigeasy@linutronix.de