docs: update stats/base/dists/gumbel examples to use Greek symbols

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---

Author: Planeshifter

lib/node_modules/@stdlib/stats/base/dists/gumbel/quantile/README.md

@@ -236,7 +236,7 @@ int main( void ) {
         mu = random_uniform( -5.0, 5.0 );
         beta = random_uniform( STDLIB_CONSTANT_FLOAT64_EPS, 20.0 );
         y = stdlib_base_dists_gumbel_quantile( p, mu, beta );
-        printf( "p: %lf, mu: %lf, beta: %lf, Q(p;mu,beta): %lf\n", p, mu, beta, y );
+        printf( "p: %lf, µ: %lf, β: %lf, Q(p;µ,β): %lf\n", p, mu, beta, y );
     }
 }
 ```

lib/node_modules/@stdlib/stats/base/dists/gumbel/quantile/examples/c/example.c

@@ -38,6 +38,6 @@ int main( void ) {
 		mu = random_uniform( -5.0, 5.0 );
 		beta = random_uniform( STDLIB_CONSTANT_FLOAT64_EPS, 20.0 );
 		y = stdlib_base_dists_gumbel_quantile( p, mu, beta );
-		printf( "p: %lf, mu: %lf, beta: %lf, Q(p;mu,beta): %lf\n", p, mu, beta, y );
+		printf( "p: %lf, µ: %lf, β: %lf, Q(p;µ,β): %lf\n", p, mu, beta, y );
 	}
 }