ACPI: Documentation: Minor spelling fix in namespace.rst

Very minor spelling fix in ACPI documentation (typo in namespace.rst).
Signed-off-by: James Pack <jpack61108@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/firmware-guide/acpi/namespace.rst

@@ -261,7 +261,7 @@ Description Tables contain information used for the creation of the
 struct acpi_device objects represented by the given row (xSDT means DSDT
 or SSDT).
 
-The forth column of the above table indicates the 'bus_id' generation
+The fourth column of the above table indicates the 'bus_id' generation
 rule of the struct acpi_device object:
 
    _HID: