chore: update docs to allow user to properly load examples

[hughhhh]

SUMMARY

For superset load-examples command to work users must create an "Admin" user with the username admin or the following error rises:

"Admin user does not exist. "
        "Please, check if test users are properly loaded "
        "(`superset load_test_users`)."

To fix this I've added a quick note to allow devs to fix the issue quickly

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #14025 (59cd39b) into master (7697bc2) will increase coverage by 2.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #14025      +/-   ##
==========================================
+ Coverage   76.87%   79.40%   +2.52%     
==========================================
  Files         936      936              
  Lines       47417    47429      +12     
  Branches     5938     5938              
==========================================
+ Hits        36454    37660    +1206     
+ Misses      10774     9643    -1131     
+ Partials      189      126      -63     

Flag	Coverage Δ
cypress	56.04% <ø> (+16.14%)	⬆️
hive	80.49% <ø> (ø)
javascript	68.86% <ø> (+0.01%)	⬆️
mysql	80.78% <ø> (ø)
postgres	80.82% <ø> (ø)
presto	80.50% <ø> (-0.01%)	⬇️
python	81.36% <ø> (-0.01%)	⬇️
sqlite	80.38% <ø> (ø)

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

Impacted Files	Coverage Δ
...rBar/CascadeFilters/CascadeFilterControl/index.tsx	85.71% <0.00%> (-6.60%)	⬇️
superset/db_engine_specs/presto.py	88.30% <0.00%> (-0.42%)	⬇️
...ontrols/AnnotationLayerControl/AnnotationLayer.jsx	84.76% <0.00%> (+0.47%)	⬆️
superset-frontend/src/components/ListView/utils.ts	85.95% <0.00%> (+0.82%)	⬆️
...nts/controls/DateFilterControl/utils/dateParser.ts	98.71% <0.00%> (+1.28%)	⬆️
superset-frontend/src/components/Select/styles.tsx	86.30% <0.00%> (+1.36%)	⬆️
...ntend/src/explore/components/ExploreChartPanel.jsx	82.60% <0.00%> (+1.44%)	⬆️
superset-frontend/src/SqlLab/reducers/sqlLab.js	40.24% <0.00%> (+1.62%)	⬆️
superset-frontend/src/chart/Chart.jsx	64.81% <0.00%> (+1.85%)	⬆️
...-frontend/src/dashboard/actions/dashboardLayout.js	98.03% <0.00%> (+1.96%)	⬆️
... and 127 more

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[betodealmeida]

Nice, thanks for fixing!