Fix mixup between mean and median (#33106)

Author: oefe

doc/source/getting_started/intro_tutorials/09_timeseries.rst

@@ -335,7 +335,7 @@ When defined, the frequency of the time series is provided by the
     <ul class="task-bullet">
         <li>
 
-Make a plot of the daily median :math:`NO_2` value in each of the stations.
+Make a plot of the daily mean :math:`NO_2` value in each of the stations.
 
 .. ipython:: python
     :okwarning:
@@ -386,4 +386,4 @@ A full overview on time series is given in the pages on :ref:`time series and da
 
 .. raw:: html
 
-   </div>
+   </div>