Fix regexp example in regular-expressions.md (#4994)

Co-authored-by: Jason Stirnaman <stirnamanj@gmail.com>

content/influxdb/v2.7/query-data/flux/regular-expressions.md

@@ -76,7 +76,7 @@ The following example drops columns whose names do not begin with `_`.
 from(bucket: "example-bucket")
     |> range(start: -15m)
     |> filter(fn: (r) => r._measurement == "mem")
-    |> drop(fn: (column) => column !~ /_.*/)
+    |> drop(fn: (column) => column !~ /^_.*/)
 ```
 
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