docs: correct location of existing time grain definition

[sfirke]

SUMMARY

config.py has obsolete path for time grain definitions, this updates two comment lines from four years ago to point to the current location.

[codecov]

Codecov Report

Merging #21309 (24a0e24) into master (fbe9807) will decrease coverage by 11.58%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #21309       +/-   ##
===========================================
- Coverage   66.45%   54.87%   -11.59%     
===========================================
  Files        1789     1789               
  Lines       68296    68296               
  Branches     7275     7275               
===========================================
- Hits        45387    37475     -7912     
- Misses      21034    28946     +7912     
  Partials     1875     1875               

Flag	Coverage Δ
hive	?
mysql	?
postgres	?
presto	52.92% <ø> (ø)
python	57.40% <ø> (-23.96%)	⬇️
sqlite	?
unit	50.77% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/config.py	90.96% <ø> (-0.65%)	⬇️
superset/utils/dashboard_import_export.py	0.00% <0.00%> (-100.00%)	⬇️
superset/key_value/commands/update.py	0.00% <0.00%> (-88.89%)	⬇️
superset/key_value/commands/delete.py	0.00% <0.00%> (-85.30%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/key_value/commands/delete_expired.py	0.00% <0.00%> (-80.77%)	⬇️
superset/dashboards/commands/importers/v0.py	15.62% <0.00%> (-76.25%)	⬇️
superset/datasets/commands/update.py	25.00% <0.00%> (-69.05%)	⬇️
superset/datasets/commands/create.py	31.25% <0.00%> (-68.75%)	⬇️
superset/datasets/commands/importers/v0.py	24.03% <0.00%> (-67.45%)	⬇️
... and 283 more

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[zhaoyongjie]

LGTM, Thanks for the fixing.