migration-5.2: fix references to BB_CURRENT_MC

Now part of the documentation.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: 2a50504bd5c211bfae26db64190863fdf53fc3c9)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/migration-5.2.rst

@@ -97,12 +97,12 @@ systemd changes
 Multiconfig changes
 ~~~~~~~~~~~~~~~~~~~
 
-The value of ``BB_CURRENT_MC`` was changed from ``default`` to an empty string
+The value of :term:`BB_CURRENT_MC` was changed from ``default`` to an empty string
 for the default multiconfig configuration to avoid needing to map the values
 within BitBake. This was already not happening in some cases so this fixes
 some obscure bugs.
 
-Any logic based on ``BB_CURRENT_MC`` equalling to ``default`` by default should
+Any logic based on :term:`BB_CURRENT_MC` equalling to ``default`` by default should
 be changed to be equal to an empty string.
 
 Virtual toolchain provider changes