Update reading-level.html

[jkshapiro]

Closes #4022 stripping out references to a formula that is no longer regarded as the preferred standard.

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[cstrobbe]

You can also remove the following links:

• Juicy Studio's Readability Test (this is a page where you can use various types of readability tests),
• Leesbaar Nederlands (which no longer has those guidelines).

[bruce-usab]

Discussed on backlog call 5/9 and concerns were raised that proposed edit might be a little too aggressive with its deletions, especially since no one on call had suggestions for replacements. Remains in drafted.