[BUG] Removed unnecessary device-to-host copy which caused a performance regression

[rlratzel]

• Removed unnecessary device-to-host copy which caused a performance regression
• Fixed some search-and-replace errors in comments
• Added more detail about k to the BC docstring

Verified performance regression was gone using the benchmarks:

> pytest -v bench_algos.py::bench_betweenness_centrality[ds=../datasets/csv/directed/cit-Patents.csv,mm=True,pa=True] --benchmark-autosave --benchmark-compare=0001

-------------------------------------------------------------------------------------------------------------------- benchmark: 2 tests ---------------------------------------------------------
Name (time in ms)                                                                                                Min                Max               Mean            StdDev            Outliers
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
bench_betweenness_centrality[ds=../datasets/csv/directed/cit-Patents.csv,mm=True,pa=True] (NOW)              30.3700 (1.0)      30.6144 (1.0)      30.4473 (1.0)      0.0798 (1.0)           5;0
bench_betweenness_centrality[ds=../datasets/csv/directed/cit-Patents.csv,mm=True,pa=True] (0001_c90f5dc)     89.8043 (2.96)     90.5968 (2.96)     90.4228 (2.97)     0.2045 (2.56)          1;1
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov-commenter]

Codecov Report

Merging #1101 into branch-0.15 will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           branch-0.15    #1101   +/-   ##
============================================
  Coverage        72.12%   72.12%           
============================================
  Files               56       56           
  Lines             2102     2102           
============================================
  Hits              1516     1516           
  Misses             586      586           

Impacted Files	Coverage Δ
...ython/cugraph/centrality/betweenness_centrality.py	98.03% <ø> (ø)

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