[Random] Remove last references to GLOBAL_RNG in seed! docstring

While the method does use `GLOBAL_RNG` by default, that's an implementation
detail, and it eventually uses `default_rng()` internally.

Author: giordano

stdlib/Random/src/Random.jl

@@ -383,8 +383,8 @@ julia> rand!(rng, zeros(5))
 rand!
 
 """
-    seed!([rng=GLOBAL_RNG], seed) -> rng
-    seed!([rng=GLOBAL_RNG]) -> rng
+    seed!([rng=default_rng()], seed) -> rng
+    seed!([rng=default_rng()]) -> rng
 
 Reseed the random number generator: `rng` will give a reproducible
 sequence of numbers if and only if a `seed` is provided. Some RNGs