Documentation/kernel-parameters: fix typo in retbleed= kernel parameter description

Fixes a typo in the retbleed= parameter description, changing
"migitation" to "mitigation".

Fixes: 7fbf47c ("x86/bugs: Add AMD retbleed= boot parameter")

Signed-off-by: Xie Yuanbin <qq570070308@gmail.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251116145302.3681-1-qq570070308@gmail.com>

Author: atomic-kernel

Documentation/admin-guide/kernel-parameters.txt

@@ -6404,7 +6404,7 @@
 			that don't.
 
 			off          - no mitigation
-			auto         - automatically select a migitation
+			auto         - automatically select a mitigation
 			auto,nosmt   - automatically select a mitigation,
 				       disabling SMT if necessary for
 				       the full mitigation (only on Zen1