chore: typo fix

Author: aayush0325

lib/node_modules/@stdlib/stats/base/dists/betaprime/stdev/README.md

@@ -175,7 +175,7 @@ for ( i = 0; i < 10; i++ ) {
 
 #### stdlib_base_dists_betaprime_stdev( alpha, beta )
 
-Returns the standard deviation of an betaprime distribution.
+Returns the standard deviation of a betaprime distribution.
 
 ```c
 double out = stdlib_base_dists_betaprime_stdev( 1.0, 3.0 );