DXCDT-244: Escape dollar sign references in prompts docs

[willvedd]

🔧 Changes

As pointed out by #332 , the custom text prompts documentation references unescaped dollar signs in the example. When Terraform plan'ing the literal example, an error similar to the following will be incurred:

│ Error: Invalid reference
│ 
│   on main.tf line 21, in resource "auth0_prompt_custom_text" "example":
│   21:         "federatedConnectionButtonText" : "Continue with ${connectionName}",
│ 
│ A reference to a resource type must be followed by at least one attribute access, specifying the resource name.

This PR simply corrects the documentation by escaping the dollar signs with another prepended dollar sign.

📚 References

Original issue: #332

Similar Stackoverflow issue: https://stackoverflow.com/questions/66953938/escaping-dollar-sign-in-terraform

🔬 Testing

Added custom text variable replacement to custom text tests. Also manually tested the example snippet.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

Base: 86.81% // Head: 86.81% // No change to project coverage 👍

Coverage data is based on head (9a85b07) compared to base (de4a0d8).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #366   +/-   ##
=======================================
  Coverage   86.81%   86.81%           
=======================================
  Files          40       40           
  Lines        8527     8527           
=======================================
  Hits         7403     7403           
  Misses        863      863           
  Partials      261      261           

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