docs: sdk-manual: appendix-obtain: fix literal block content

Restore the error message as it is supposed to be issued.

The literal block renders its content verbatim so a term role (`:term:`)
would appear as :term: string in there and `---` as --- string instead
of the expected hyphen.

Fixes: e319b3bf4eb6 ("manuals: add missing references to SDKMACHINE and SDK_ARCH")
Cc: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: 95158beba0aea7539024688c71b54fed5267faec)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/sdk-manual/appendix-obtain.rst

@@ -140,9 +140,10 @@ build the SDK installer. Follow these steps:
       message similar to the following::
 
          The extensible SDK can currently only be built for the same
-         architecture as the machine being built on --- :term:`SDK_ARCH`
-         is set to ``i686`` (likely via setting :term:`SDKMACHINE`) which is
-         different from the architecture of the build machine (``x86_64``).
+         architecture as the machine being built on - SDK_ARCH
+         is set to i686 (likely via setting SDKMACHINE) which is
+         different from the architecture of the build machine (x86_64).
+         Unable to continue.
 
 
 #. *Build the SDK Installer:* To build the SDK installer for a standard