std.rand: make weightedIndex proportions param a const slice

The function does not mutate the proportions and the signature should reflect that.

lib/std/rand.zig

@@ -343,7 +343,7 @@ pub const Random = struct {
     ///
     /// This is useful for selecting an item from a slice where weights are not equal.
     /// `T` must be a numeric type capable of holding the sum of `proportions`.
-    pub fn weightedIndex(r: std.rand.Random, comptime T: type, proportions: []T) usize {
+    pub fn weightedIndex(r: std.rand.Random, comptime T: type, proportions: []const T) usize {
         // This implementation works by summing the proportions and picking a random
         //  point in [0, sum).  We then loop over the proportions, accumulating
         //  until our accumulator is greater than the random point.

lib/std/rand/test.zig

@@ -452,7 +452,7 @@ test "Random weightedIndex" {
         var prng = DefaultPrng.init(0);
         const random = prng.random();
 
-        var proportions = [_]T{ 2, 1, 1, 2 };
+        const proportions = [_]T{ 2, 1, 1, 2 };
         var counts = [_]f64{ 0, 0, 0, 0 };
 
         const n_trials: u64 = 10_000;