docs: fix incorrect return value in C usage section triangular/mean

PR-URL: #10958

Reviewed-by: Philipp Burckhardt <pburckhardt@outlook.com>

Author: Om-A-osc

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

@@ -173,7 +173,7 @@ Returns the [expected value][expected-value] of a [triangular][triangular-distri
 
 ```c
 double out = stdlib_base_dists_triangular_mean( 0.0, 1.0, 0.5 );
-// returns ~0.333
+// returns 0.5
 ```
 
 The function accepts the following arguments: