docs(schema): fix leftover Italian "Chiave" in English schema header

Fixes #313. The English schema reference (published at
https://yml.publiccode.tools/country.html from
docs/standard/schema.it.rst) had one section header using the
Italian word "Chiave" while the other 11 sibling entries in the
same file use "Key". This was the only leftover Italian on line
130.

Also adjusts the RST underline length so it matches the new
heading length (6-char reduction, RST requires underline >= title
length).

No changes to docs/standard/schema.core.rst or country.rst.

Author: tmchow

docs/standard/schema.it.rst

@@ -127,8 +127,8 @@ Section ``riuso``
 This section contains a set of keys related to the publication of the software
 inside the reuse catalog of `Developers Italia <https://developers.italia.it>`__.
 
-Chiave ``riuso/codiceIPA`` (*deprecated*)
-'''''''''''''''''''''''''''''''''''''''''
+Key ``riuso/codiceIPA`` (*deprecated*)
+''''''''''''''''''''''''''''''''''''''
 
 -  Type: string (iPA code) 
 -  Presence: mandatory if ``repoOwner`` is a Public Administration