MINOR: [Python][Docs] Fix mention of default for coerce_timestamps keyword in reading Parquet (apache#41498)

### Rationale for this change

In the meantime we have updated the default for the `version` keyword from "1.0" to "2.4" and recently to "2.6", but this was not yet reflected in the description of the `coerce_timestamps` keyword.

Authored-by: Joris Van den Bossche <[email protected]>
Signed-off-by: Joris Van den Bossche <[email protected]>

Author: jorisvandenbossche

python/pyarrow/parquet/core.py

@@ -761,9 +761,9 @@ def _sanitize_table(table, new_schema, flavor):
     by flavor argument. This take priority over the coerce_timestamps option.
 coerce_timestamps : str, default None
     Cast timestamps to a particular resolution. If omitted, defaults are chosen
-    depending on `version`. By default, for ``version='1.0'`` (the default)
-    and ``version='2.4'``, nanoseconds are cast to microseconds ('us'), while
-    for other `version` values, they are written natively without loss
+    depending on `version`. For ``version='1.0'`` and ``version='2.4'``,
+    nanoseconds are cast to microseconds ('us'), while for
+    ``version='2.6'`` (the default), they are written natively without loss
     of resolution.  Seconds are always cast to milliseconds ('ms') by default,
     as Parquet does not have any temporal type with seconds resolution.
     If the casting results in loss of data, it will raise an exception