DOC: Fix num2date docstring

There is no addition of one anymore.

Author: StefRe

lib/matplotlib/dates.py

@@ -525,8 +525,7 @@ def num2date(x, tz=None):
 
     Notes
     -----
-    The addition of one here is a historical artifact. Also, note that the
-    Gregorian calendar is assumed; this is not universal practice.
+    The Gregorian calendar is assumed; this is not universal practice.
     For details, see the module docstring.
     """
     tz = _get_tzinfo(tz)