migration-guides: add missing opening tag colon

As reported by "make sphinx-lint"

(From yocto-docs rev: 7e8d8d6d47da61457798e90b1d53f3082fe76668)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/migration-4.2.rst

@@ -121,7 +121,7 @@ Removed variables
 The following variables have been removed:
 
 -  ``SERIAL_CONSOLE``, deprecated since version 2.6, replaced by :term:`SERIAL_CONSOLES`.
--  ``PACKAGEBUILDPKGD``, a mostly internal variable in the ref:`ref-classes-package`
+-  ``PACKAGEBUILDPKGD``, a mostly internal variable in the :ref:`ref-classes-package`
    class was rarely used to customise packaging. If you were using this in your custom
    recipes or bbappends, you will need to switch to using :term:`PACKAGE_PREPROCESS_FUNCS`
    or :term:`PACKAGESPLITFUNCS` instead.

documentation/migration-guides/release-notes-4.0.4.rst

@@ -247,7 +247,7 @@ poky
 
 openembedded-core
 
--  Repository Location: oe_git:`/openembedded-core`
+-  Repository Location: :oe_git:`/openembedded-core`
 -  Branch: :oe_git:`kirkstone </openembedded-core/log/?h=kirkstone>`
 -  Tag:  :oe_git:`yocto-4.0.4 </openembedded-core/log/?h=yocto-4.0.4>`
 -  Git Revision: :oe_git:`f7766da462905ec67bf549d46b8017be36cd5b2a </openembedded-core/commit/?id=f7766da462905ec67bf549d46b8017be36cd5b2a>`