Manual: fix typo

[skip ci]

(cherry picked from commit 7e894404)

doc/manual/source/the-model-file.rst

@@ -2048,7 +2048,7 @@ in this case ``initval`` is used to specify the terminal conditions.
     .. option:: last_obs = {INTEGER | DATE}
-        The observaton number or the date (see
+        The observation number or the date (see
 	:ref:`dates-members`) of the last observation to be used in
 	the file.