Remove string-functions.com from KnownIssueJsonStepByStep.md (#15865)

Author: akoeplinger

Documentation/Projects/Build Analysis/KnownIssueJsonStepByStep.md

@@ -6,7 +6,7 @@ This is a summarized version of our documentation, you can always look at [Compl
     - "ErrorMessage" works as `contains` (single line, case-insensitive)
     - "ErrorPattern" works as `regex` (single line, case-insensitive, no backtracking)
 1. Write the error message: <sup>  Additional documentation on [how to write an error message](https://github.com/dotnet/arcade/blob/main/Documentation/Projects/Build%20Analysis/KnownIssues.md#filling-out-known-issues-json-blob) or [known issues with a list of errors](https://github.com/dotnet/arcade/blob/main/Documentation/Projects/Build%20Analysis/KnownIssues.md#filling-out-known-issues-with-a-list-of-errors)</sup>
-    - If you use “ErrorMessage” select the part of the line that you want to match, use [string-functions](https://string-functions.com/countsubstrings.aspx) to test it.
+    - If you use “ErrorMessage” select the part of the line that you want to match.
     - If you use "ErrorPattern" write the regex that matches your error message. Use [regex101](https://regex101.com/) (choose `.NET (C#)` flavor with `Single line`, `Insensitive`, `No backtracking` flags enabled) to test it.
     - To escape special characters in json, use: [freeformatter](https://www.freeformatter.com/json-escape.html).
 1. Set BuildRetry to `true` if you would like to enable automatic build retries for any build that matches the error.
@@ -103,4 +103,4 @@ D:\a\1\s\src\Directory.Build.props(3,3): warning : Failed to retrieve informatio
 
 **Explanation:** This is a flaky failure that is related to a connection issue. It’s highly likely that the problem will be resolved by just retrying the build, so we set BuildRetry to true.
 
-</details>
+</details>