Make QT_API a link in the qt embedding example.

(This is already the case elsewhere in the docs.)

Author: anntzer

examples/user_interfaces/embedding_in_qt_sgskip.py

@@ -5,8 +5,8 @@
 
 Simple Qt application embedding Matplotlib canvases.  This program will work
 equally well using any Qt binding (PyQt6, PySide6, PyQt5, PySide2).  The
-binding can be selected by setting the ``QT_API`` environment variable to the
-binding name, or by first importing it.
+binding can be selected by setting the :envvar:`QT_API` environment variable to
+the binding name, or by first importing it.
 """
 
 import sys