Fix ...diagnostics.deprecated... rule ID

In #1560, in commit 4ef5f5d, we
renamed the rule IDs in the diagnostics chapter, except
those for the `diagnostic` tool attribute namespace, to the
plural form, `attributes.diagnostics.*`.  But we missed one,
`attributes.diagnostics.deprecated.allowed-positions` (as
corrected).  Let's fix that.

Author: traviscross

src/attributes/diagnostics.md

@@ -321,7 +321,7 @@ The `deprecated` attribute has several forms:
     message. This is typically used to provide an explanation about the
     deprecation and preferred alternatives.
 
-r[attributes.diagnostic.deprecated.allowed-positions]
+r[attributes.diagnostics.deprecated.allowed-positions]
 The `deprecated` attribute may be applied to any [item], [trait item], [enum
 variant], [struct field], [external block item], or [macro definition]. It
 cannot be applied to [trait implementation items][trait-impl]. When applied to an item