doc: releases: Fix GH link in migration guide v3.6

Putting a space between :github: and the PR number causes the rendered
text to not link to the issue.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
(cherry picked from commit 5a49b7c90d8780e2a145b4cddab04f35b1d6c21a)
diff --git a/doc/releases/migration-guide-3.6.rst b/doc/releases/migration-guide-3.6.rst
index 8d8a2ac..ffe9326 100644
--- a/doc/releases/migration-guide-3.6.rst
+++ b/doc/releases/migration-guide-3.6.rst
@@ -638,7 +638,7 @@
 * :kconfig:option:`CONFIG_SHELL_BACKEND_SERIAL_API` now does not automatically default to
   :kconfig:option:`CONFIG_SHELL_BACKEND_SERIAL_API_ASYNC` when
   :kconfig:option:`CONFIG_UART_ASYNC_API` is enabled, :kconfig:option:`CONFIG_SHELL_ASYNC_API`
-  also has to be enabled in order to use the asynchronous serial shell (:github: `68475`).
+  also has to be enabled in order to use the asynchronous serial shell (:github:`68475`).
 
 ZBus
 ====