docs: fix multiple doc build warnings in enumeration.rst

The conversion of acpi/enumeration.txt to RST included one markup error,
leading to many warnings like:

  .../firmware-guide/acpi/enumeration.rst:430: WARNING: Unexpected indentation.

Add the missing colon and create some peace.

Fixes: c24bc66e ("Documentation: ACPI: move enumeration.txt to firmware-guide/acpi and convert to reST")
Cc: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/firmware-guide/acpi/enumeration.rst

@@ -423,7 +423,7 @@ will be enumerated to depends on the device ID returned by _HID.
 
 For example, the following ACPI sample might be used to enumerate an lm75-type
 I2C temperature sensor and match it to the driver using the Device Tree
-namespace link:
+namespace link::
 
 	Device (TMP0)
 	{