Fix heroku guide to set database settings in proper env path

[derrickmehaffy]

Signed-off-by: Derrick Mehaffy derrickmehaffy@gmail.com

Description of what you did:

closes #8164

The documentation was not clear as it was replacing the SQLite config, making it difficult to understand for new users how to run locally.

[codecov]

Codecov Report

Merging #8165 into documentation will increase coverage by 13.64%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           documentation    #8165       +/-   ##
==================================================
+ Coverage          19.14%   32.78%   +13.64%     
==================================================
  Files                855     1197      +342     
  Lines              11933    13020     +1087     
  Branches            1898     1286      -612     
==================================================
+ Hits                2284     4268     +1984     
+ Misses              8100     7907      -193     
+ Partials            1549      845      -704     

Flag	Coverage Δ
#front	24.81% <ø> (+10.32%)	⬆️
#unit	53.88% <ø> (+13.94%)	⬆️

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

Impacted Files	Coverage Δ
packages/strapi-plugin-email/admin/src/pluginId.js	0.00% <0.00%> (-100.00%)	⬇️
...api-plugin-users-permissions/admin/src/pluginId.js	0.00% <0.00%> (-100.00%)	⬇️
...n/src/containers/NotificationProvider/constants.js	0.00% <0.00%> (-100.00%)	⬇️
...n/admin/src/containers/LanguageProvider/reducer.js	0.00% <0.00%> (-61.54%)	⬇️
packages/strapi/lib/services/metrics/index.js	65.00% <0.00%> (-35.00%)	⬇️
...admin/admin/src/components/ContainerFluid/index.js	75.00% <0.00%> (-25.00%)	⬇️
...trapi-admin/admin/src/containers/LeftMenu/index.js	42.30% <0.00%> (-24.36%)	⬇️
...admin/src/containers/LanguageProvider/selectors.js	50.00% <0.00%> (-21.43%)	⬇️
...in/admin/src/containers/Webhooks/EditView/index.js	20.33% <0.00%> (-18.07%)	⬇️
...-admin/admin/src/containers/HomePage/components.js	40.00% <0.00%> (-17.15%)	⬇️
... and 1269 more

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