docs(CHANGES): Fix heading level for Breaking changes section

why: "## Breaking changes" at line 60 was same level as release headers,
creating an orphaned section outside the 0.48.0 release.
what:
- Change "## Breaking changes" to "### Breaking changes" to nest under 0.48.0
- Update "### Renamed commands" to "#### Renamed commands" accordingly

Author: tony

CHANGES

@@ -57,9 +57,9 @@ release. Set `LIBTMUX_SUPPRESS_VERSION_WARNING=1` to suppress the warning.
 
 Added tmux 3.6 to test grid and `TMUX_MAX_VERSION` 3.4 -> 3.6.
 
-## Breaking changes
+### Breaking changes
 
-### Renamed commands
+#### Renamed commands
 
 - Renamed `Window.set_window_option()` to {meth}`Window.set_option()` (#516)