Fix tutorial

[Johnson-Wang]

• Fix itemize issues
• Fix some typos.

[codecov]

Codecov Report

Merging #3658 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3658      +/-   ##
==========================================
+ Coverage   61.68%   61.71%   +0.03%     
==========================================
  Files         207      207              
  Lines       14067    14067              
  Branches     2378     2378              
==========================================
+ Hits         8677     8682       +5     
  Misses       4947     4947              
+ Partials      443      438       -5     

Flag	Coverage Δ
#unittests	61.71% <ø> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/test_mixins.py	53.76% <0.00%> (+3.22%)	⬆️
mmdet/models/dense_heads/rpn_test_mixin.py	83.87% <0.00%> (+6.45%)	⬆️

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