DOC: Do not list deprecated aliases in docs

The PRs #55792 (Y), #52064 (Q), and #55553 (M) deprecated the single letter
version of the aliases in favour of the -end version of them.

This table was missed as part of that work.

Author: tacaswell

doc/source/user_guide/timeseries.rst

@@ -1317,9 +1317,9 @@ frequencies. We will refer to these aliases as *period aliases*.
     "B", "business day frequency"
     "D", "calendar day frequency"
     "W", "weekly frequency"
-    "M", "monthly frequency"
-    "Q", "quarterly frequency"
-    "Y", "yearly frequency"
+    "ME", "monthly frequency"
+    "QE", "quarterly frequency"
+    "YE", "yearly frequency"
     "h", "hourly frequency"
     "min", "minutely frequency"
     "s", "secondly frequency"
@@ -1332,6 +1332,8 @@ frequencies. We will refer to these aliases as *period aliases*.
    Aliases ``H``, ``T``, ``S``, ``L``, ``U``, and ``N`` are deprecated in favour of the aliases
    ``h``, ``min``, ``s``, ``ms``, ``us``, and ``ns``.
 
+   Aliases ``Y``, ``M``, and ``Q`` are deprecated in favour of the aliases ``YE``, ``ME``, ``QE``.
+
 
 Combining aliases
 ~~~~~~~~~~~~~~~~~