rawmoment: update spec

Author: jvdp1

src/stdlib_experimental_stats.md

@@ -19,7 +19,7 @@ Returns the mean of all the elements of `array`, or of the elements of `array` a
 ### Arguments
 
 `array`: Shall be an array of type `integer`, `real`, or `complex`.
-[, center [,
+
 `dim`: Shall be a scalar of type `integer` with a value in the range from 1 to n, where n is the rank of `array`.
 
 `mask` (optional): Shall be of type `logical` and either by a scalar or an array of the same shape as `array`.
@@ -85,7 +85,7 @@ The _k_-th order central moment is defined as :
 
 `dim`: Shall be a scalar of type `integer` with a value in the range from 1 to n, where n is the rank of `array`.
 
-`center` (optional): Shall be a scalar or an array of the same type of `array`, if `array` is `real` or `complex`, or a scalar or an array of type `real(dp)` if `array` is of type `integer`.
+`center` (optional): Shall be a scalar of the same type of `array` if `array` is `real` or `complex`, or of type `real(dp)` if `array` is of type `integer`. If `dim` is provided, `center` shall be an array (with a shape similar to that of `array` with dimension `dim` dropped) of the same type of `array` if `array` is `real` or `complex`, or of type `real(dp)` if `array` is of type `integer`.
 
 `mask` (optional): Shall be of type `logical` and either by a scalar or an array of the same shape as `array`.