Fix incorrect doc for to_datetime (#28670)

Author: harupy

pandas/core/tools/datetimes.py

@@ -637,7 +637,7 @@ def to_datetime(
         datetime strings, and if it can be inferred, switch to a faster
         method of parsing them. In some cases this can increase the parsing
         speed by ~5-10x.
-    origin : scalar, default is 'unix'
+    origin : scalar, default 'unix'
         Define the reference date. The numeric values would be parsed as number
         of units (defined by `unit`) since this reference date.