doc: fix several typos in changelog entries

After this change the release notes are able to be built.

Changed '---' to '----', so comments for a release manager are parsed
correctly.

changelogs/unreleased/change-transaction-behaviour-after-rollback-on-yield.md

-## core/feature
+## feature/core
 
 * Previously, if a yield occurs for a transaction that does not
   support it, we roll back all its statements, but still process

changelogs/unreleased/drop-net-box-console.md

@@ -3,5 +3,6 @@
 * **[Breaking change]** net.box console support, which was marked deprecated
   in 1.10, was dropped. Use `require('console').connect()` instead.
 
----
+----
+
 Breaking change: net.box console support was dropped.

changelogs/unreleased/gh-5814-memprof-group-allocation-on-traces.md

-##feature/luajit
+## feature/luajit
 
 * Now memory profiler records allocations from traces grouping them by the
   trace number (gh-5814). The memory profiler parser can display the new type

changelogs/unreleased/gh-6242-net-box-drop-conn-timeout.md

@@ -5,5 +5,6 @@
   it negatively affected performance of hot net.box methods, like call() and
   select(), in case those are called without specifying a timeout (gh-6242).
 
----
+----
+
 Breaking change: timeout() method of net.box connection was dropped.