docs: fix the explanation about XPath variables in syntax doc

[hnakamur]

Related:

• XML://key not work · Issue #1322 · corazawaf/coraza
• Difference in JSON array element notation between Coraza (.${index}) and ModSecurity (.array_${index}) · corazawaf/coraza · Discussion #1426

Implementation:

• https://github.com/corazawaf/coraza/blob/v3.3.3/internal/bodyprocessors/xml.go#L23-L24
• https://github.com/corazawaf/coraza/blob/v3.3.3/internal/bodyprocessors/json.go#L69-L75

[fzipi]

CC @jptosso can you quickly review?