docs: add secret scheme details to the example

[aviadatsnyk]

What this PR does / why we need it:
I'm not sure if this is a docs fix or a bug report (but I think it's the former): if you create a basic-auth secret from a file not named auth (i.e myauth.htpasswd) - the ingress controller returns a 503 (not writing anything to the error log, at least not on the default settings).

Special notes for your reviewer:
If this is a bug and not a docs issue, or I missed the relevant docs - sorry.

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[codecov-io]

Codecov Report

Merging #2583 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2583   +/-   ##
=======================================
  Coverage   40.54%   40.54%           
=======================================
  Files          75       75           
  Lines        5105     5105           
=======================================
  Hits         2070     2070           
  Misses       2754     2754           
  Partials      281      281

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[aviadatsnyk]

I signed the CLA

[aledbf]

/lgtm
/approve

[aledbf]

@aviadatsnyk thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, aviadatsnyk

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