docs: Incorrect regexp flag

[jseldess]

Re: Functions and Operators

Issue Description

For the regexp_replace(input: string, regex: string, replace: string, flags: string) function, the supported flags are listed in a table, and the flag for "Global matching" is given as "i", but this is incorrect; the correct flag for this option/mode is "g", as I have discovered while testing a query.

Suggested Resolution

Change "i" to "g". 😺

Moved from cockroachdb/docs#3312 since the docs a derived from the cockroach code.

[jseldess]

Looks like the flag was (inadvertently) changed from "g" to "i" here: 88a3e49#diff-a78b90ae27ed7c44b4c7c4ceaa691cdbL775