Rename MultiClassificationObjective to MulticlassClassificationObjective

[dsherry]

Changes:

• Renamed MultiClassificationObjective to MulticlassClassificationObjective, to align with pipeline naming scheme
• Remove hyphen from multiclass in two places

[codecov]

Codecov Report

Merging #715 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #715   +/-   ##
=======================================
  Coverage   99.21%   99.21%           
=======================================
  Files         140      140           
  Lines        4985     4985           
=======================================
  Hits         4946     4946           
  Misses         39       39           

Impacted Files	Coverage Δ
evalml/objectives/__init__.py	100.00% <100.00%> (ø)
.../objectives/multiclass_classification_objective.py	100.00% <100.00%> (ø)
evalml/objectives/standard_metrics.py	100.00% <100.00%> (ø)
...pelines/classification/random_forest_multiclass.py	100.00% <100.00%> (ø)

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[angela97lin]

LGTM! Thanks for this; I had considered MulticlassClassificationObjective but it just seemed too long, lolol

[dsherry]

@angela97lin no prob! Yeah I just noticed this and figured it would be good to have the names align before we do a release, and wanted to hear what you all thought about it. Sounds good, will merge!