updated haiku guide with new JAX RNG api

[chiamp]

updated haiku guide, after #3337 changed Flax to use the new Jax RNG api

[codecov-commenter]

Codecov Report

Merging #3343 (089f237) into main (d9f83bf) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #3343   +/-   ##
=======================================
  Coverage   82.64%   82.64%           
=======================================
  Files          55       55           
  Lines        6363     6363           
=======================================
  Hits         5259     5259           
  Misses       1104     1104           

Files Changed	Coverage Δ
flax/core/meta.py	95.40% <ø> (ø)
flax/core/scope.py	88.80% <ø> (ø)
flax/cursor.py	95.83% <ø> (ø)
flax/errors.py	86.48% <ø> (ø)
flax/linen/initializers.py	100.00% <ø> (ø)
flax/linen/module.py	92.02% <ø> (ø)
flax/linen/normalization.py	96.89% <ø> (ø)
flax/linen/summary.py	98.28% <ø> (ø)
flax/linen/transforms.py	94.14% <ø> (ø)
flax/linen/recurrent.py	97.30% <100.00%> (ø)

[IvyZX]

Thanks for fixing it!