update CONTRIBUTING.md

[vivian1912]

What does this PR do?
update commit message section of contribution doc
Why are these changes required?
make it more clear to illustrate how to write a committing message for a pull request
This PR has been tested by:

• Unit Tests
• Manual Testing

Follow up

Extra details

[codecov-commenter]

Codecov Report

Merging #4604 (b2782b3) into develop (7f3ed0a) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop    #4604      +/-   ##
=============================================
- Coverage      54.24%   54.19%   -0.06%     
+ Complexity      7946     7938       -8     
=============================================
  Files            831      831              
  Lines          47120    47120              
  Branches        5103     5103              
=============================================
- Hits           25562    25537      -25     
- Misses         19528    19555      +27     
+ Partials        2030     2028       -2     

Impacted Files	Coverage Δ
...n/common/net/udp/message/discover/PongMessage.java	0.00% <0.00%> (-90.00%)	⬇️
...mon/overlay/discover/table/DistanceComparator.java	80.00% <0.00%> (-20.00%)	⬇️
...tron/common/overlay/discover/node/NodeHandler.java	51.56% <0.00%> (-9.38%)	⬇️
...udp/message/discover/DiscoverMessageInspector.java	21.62% <0.00%> (-8.11%)	⬇️
.../java/org/tron/common/net/udp/message/Message.java	56.52% <0.00%> (-4.35%)	⬇️
.../tron/common/overlay/discover/table/NodeTable.java	90.76% <0.00%> (-3.08%)	⬇️
...ay/discover/node/statistics/MessageStatistics.java	91.24% <0.00%> (-2.92%)	⬇️
...tron/common/overlay/discover/node/NodeManager.java	73.68% <0.00%> (ø)
...mework/src/main/java/org/tron/core/db/Manager.java	55.99% <0.00%> (+0.62%)	⬆️
...re/metrics/blockchain/BlockChainMetricManager.java	70.00% <0.00%> (+2.50%)	⬆️
... and 2 more

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