[BUG] spelling mistake in time_series.py
**Describe the bug**
fill_missing_date->fill_missing_dates

as people are cut and paste from the error, this is a bit annoying

**To Reproduce**
Steps to reproduce the behavior, preferably code snippet.

**Expected behavior**
A clear and concise description of what you expected to happen.

**System (please complete the following information):**
 - Python version: [e.g. 3.7]
 - darts version [e.g. 0.5.0]

**Additional context**
Add any other context about the problem here.

diff --git a/darts/timeseries.py b/darts/timeseries.py
index 69e7d348..3fc6249e 100644
--- a/darts/timeseries.py
+++ b/darts/timeseries.py
@@ -85,7 +85,7 @@ class TimeSeries:
                      'The time index of the provided DataArray is missing the freq attribute, and the frequency could '
                      'not be directly inferred. '
                      'This probably comes from inconsistent date frequencies with missing dates. '
-                     'If you know the actual frequency, try setting `fill_missing_date=True, freq=actual_frequency`. '
+                     'If you know the actual frequency, try setting `fill_missing_dates=True, freq=actual_frequency`. '
                      'If not, try setting `fill_missing_dates=True, freq=None` to see if a frequency can be inferred.',
                      logger)
 
['darts/timeseries.py']