Small fixes to development_guide: title and label names

* Fix title to use a proper "title" section marker
* Fix labels by adding a " " after the ":"
* Fix OS labels after obvious mishap
* Sort labels

doc/en/development_guide.rst

@@ -1,5 +1,6 @@
+=================
 Development Guide
------------------
+=================
 
 Some general guidelines regarding development in pytest for core maintainers and general contributors. Nothing here
 is set in stone and can't be changed, feel free to suggest improvements or changes in the workflow.
@@ -41,8 +42,8 @@ Here is a list of labels and a brief description mentioning their intent.
 
 **Type**
 
-* ``type:bug``: problem that needs to be addressed.
 * ``type: backward compatibility``: issue that will cause problems with old pytest versions.
+* ``type: bug``: problem that needs to be addressed.
 * ``type: deprecation``: feature that will be deprecated in the future.
 * ``type: docs``: documentation missing or needing clarification.
 * ``type: enhancement``: new feature or API change, should be merged into ``features``.
@@ -66,17 +67,21 @@ Here is a list of labels and a brief description mentioning their intent.
 * ``topic: collection``
 * ``topic: fixtures``
 * ``topic: parametrize``
-* ``topic: selection``
 * ``topic: reporting``
+* ``topic: selection``
 * ``topic: tracebacks``
 
 **Plugin (internal or external)**
 
-* ``plugin: junitxml``
 * ``plugin: cache``
 * ``plugin: capture``
 * ``plugin: doctests``
+* ``plugin: junitxml``
+* ``plugin: monkeypatch``
+* ``plugin: nose``
+* ``plugin: pastebin``
 * ``plugin: pytester``
+* ``plugin: tmpdir``
 * ``plugin: unittest``
 * ``plugin: warnings``
 * ``plugin: xdist``
@@ -87,9 +92,9 @@ Here is a list of labels and a brief description mentioning their intent.
 Issues specific to a single operating system. Do not use as a means to indicate where an issue originated from, only
 for problems that happen **only** in that system.
 
+* ``os: linux``
+* ``os: mac``
 * ``os: windows``
-* ``linux: windows``
-* ``mac: windows``
 
 **Temporary**