*: tiny update CONTRIBUTING.md

[tiancaiamao]

What problem does this PR solve?

Review CONTRIBUTING.md, tiny update.

What is changed and how it works?

After we migrate to Go module, the Makefile still there and some tools depends on GOPATH.
So we haven't get rid of GOPATH totally.

Check List

• No code

---

This change is 

[tiancaiamao]

PTAL @shenli

[shenli]

@lilin90 PTAL

[tiancaiamao]

/run-all-tests

[tiancaiamao]

/run-unit-test

[tiancaiamao]

PTAL @shenli @lilin90

[lilin90]

LGTM

[tiancaiamao]

PTAL @shenli

[tiancaiamao]

PTAL @shenli

[tiancaiamao]

PTAL @shenli

[zz-jason]

LGTM

[codecov-io]

Codecov Report

Merging #8814 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8814      +/-   ##
==========================================
+ Coverage   67.24%   67.25%   +0.01%     
==========================================
  Files         371      371              
  Lines       75972    75972              
==========================================
+ Hits        51086    51094       +8     
+ Misses      20372    20367       -5     
+ Partials     4514     4511       -3

Impacted Files	Coverage Δ
executor/join.go	78.44% <0%> (+0.51%)	⬆️
ddl/delete_range.go	74.28% <0%> (+1.71%)	⬆️
ddl/session_pool.go	93.1% <0%> (+10.34%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4b98ad6...193c2bc. Read the comment docs.