Fix SQL syntax for error rate calculation in Analytics Engine

[sofiyevsr]

Summary

Fixes an invalid SQL query example in the Workers Analytics Engine documentation (references/analytics-engine/api.md).

The Workers Analytics Engine SQL API is built on a restricted time-series SQL engine that does not support standard SQL CASE WHEN ... THEN ... END expressions.
Attempting to execute a query using CASE WHEN results in an HTTP 422 error:

  Error: Analytics API error (422): Input was invalid: unsupported expression type: CASE WHEN ...

The supported syntax for conditional logic in Workers Analytics Engine SQL is the if(condition, true_value, false_value) scalar function.

Changes

• Updated the Error rate example in references/analytics-engine/api.md:

    -- Before
    SUM(CASE WHEN blob3 LIKE '5%' THEN 1 ELSE 0 END) AS errors
  
    -- After
    SUM(if(blob3 LIKE '5%', 1, 0)) AS errors

References

• Cloudflare Workers Analytics Engine SQL Reference (https://developers.cloudflare.com/analytics/analytics-engine/sql-reference/conditional-functions/)