fix typo (#1155)

datetimes.qmd

@@ -456,7 +456,7 @@ hour(datetime) <- hour(datetime) + 1
 datetime
 ```
 
-Alternatively, rather than modifying an existing variabke, you can create a new date-time with `update()`.
+Alternatively, rather than modifying an existing variable, you can create a new date-time with `update()`.
 This also allows you to set multiple values in one step:
 
 ```{r}