Fix typo in heuristics of BracketedSort (#52494)

The impact of this typo was a) massively decreased performance that was
b) predicted by heuristic dispatch, resulting in this algorithm not
being dispatched too. I introduced this typo in
187e8c2 after performing all the benchmarking and before merging.

base/sort.jl

@@ -1262,7 +1262,7 @@ function _sort!(v::AbstractVector, a::BracketedSort, o::Ordering, kw)
     k = cbrt(ln)
     k2 = round(Int, k^2)
     k2ln = k2/ln
-    offset = .15k2*top_set_bit(k2) # TODO for further optimization: tune this
+    offset = .15k*top_set_bit(k2) # TODO for further optimization: tune this
     lo_signpost_i, hi_signpost_i =
         (floor(Int, (tar - lo) * k2ln + lo + off) for (tar, off) in
             ((minimum(target), -offset), (maximum(target), offset)))

test/sorting.jl

@@ -721,10 +721,9 @@ end
     for alg in safe_algs
         @test sort(1:n, alg=alg, lt = (i,j) -> v[i]<=v[j]) == perm
     end
-    # This could easily break with minor heuristic adjustments
-    # because partialsort is not even guaranteed to be stable:
-    @test partialsort(1:n, 172, lt = (i,j) -> v[i]<=v[j]) == perm[172]
-    @test partialsort(1:n, 315:415, lt = (i,j) -> v[i]<=v[j]) == perm[315:415]
+    # Broken by the introduction of BracketedSort in #52006 which is unstable
+    @test_broken partialsort(1:n, 172, lt = (i,j) -> v[i]<=v[j]) == perm[172]
+    @test_broken partialsort(1:n, 315:415, lt = (i,j) -> v[i]<=v[j]) == perm[315:415]
 
     # lt can be very poorly behaved and sort will still permute its input in some way.
     for alg in safe_algs