Documentation: ACPI: EINJ: Fix obsolete example

Since commit 488dac0c ("libfs: fix error cast of negative value in
simple_attr_write()"), the EINJ debugfs interface no longer accepts
negative values as input. Attempt to do so will result in EINVAL.

Fixes: 488dac0c ("libfs: fix error cast of negative value in simple_attr_write()")
Signed-off-by: Qifu Zhang <zhangqifu@bytedance.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/firmware-guide/acpi/apei/einj.rst

@@ -168,7 +168,7 @@ An error injection example::
   0x00000008	Memory Correctable
   0x00000010	Memory Uncorrectable non-fatal
   # echo 0x12345000 > param1		# Set memory address for injection
-  # echo $((-1 << 12)) > param2		# Mask 0xfffffffffffff000 - anywhere in this page
+  # echo 0xfffffffffffff000 > param2		# Mask - anywhere in this page
   # echo 0x8 > error_type			# Choose correctable memory error
   # echo 1 > error_inject			# Inject now