docs: update README for cosine mean function with correct usage and parameters

Author: sethdivyansh

lib/node_modules/@stdlib/stats/base/dists/cosine/mean/README.md

@@ -154,25 +154,25 @@ for ( i = 0; i < 10; i++ ) {
 ### Usage
 
 ```c
-#include "stdlib/stats/base/dists/arcsine/variance.h"
+#include "stdlib/stats/base/dists/cosine/mean.h"
 ```
 
-#### stdlib_base_dists_arcsine_variance( a, b )
+#### stdlib_base_dists_cosine_mean( a, b )
 
-Returns the variance of an arcsine distribution.
+Returns the mean of an cosine distribution.
 
 ```c
-double out = stdlib_base_dists_arcsine_variance( 0.0, 1.0 );
-// returns ~0.125
+double out = stdlib_base_dists_cosine_mean( 2.0, 1.0 );
+// returns 2.0
 ```
 
 The function accepts the following arguments:
 
--   **a**: `[in] double` minimum support.
--   **b**: `[in] double` maximum support.
+-   **mu**: `[in] double` location parameter.
+-   **s**: `[in] double` scale parameter.
 
 ```c
-double stdlib_base_dists_arcsine_variance( const double a, const double b );
+double stdlib_base_dists_cosine_mean( const double mu, const double s );
 ```
 
 </section>