spec_md: remove mean from moment example

Author: jvdp1

src/stdlib_experimental_stats.md

@@ -102,7 +102,7 @@ If `mask` is specified, the result is the _k_-th  (central) moment of all elemen
 
 ```fortran
 program demo_moment
-    use stdlib_experimental_stats, only: mean, moment
+    use stdlib_experimental_stats, only: moment
     implicit none
     real :: x(1:6) = [ 1., 2., 3., 4., 5., 6. ]
     real :: y(1:2, 1:3) = reshape([ 1., 2., 3., 4., 5., 6. ], [ 2, 3])