doc: Fix some descriptions related to pg_ident_file_mappings

pg_ident_file_mappings.line_number was described as a line number in
pg_ident.conf for a "rule" number, but this should refer to a "map".
The same inconsistent term was used in the main paragraph describing the
view.

Extracted from a patch by the same author.  Issue introduced by
a2c8499 where this view has been added.

Author: Julien Rouhaud
Discussion: https://postgr.es/m/20221026031948.cbrnzgy5e7glsq2d@jrouhaud
Backpatch-through: 15

doc/src/sgml/system-views.sgml

@@ -1112,7 +1112,7 @@
    of the contents of the client user name mapping configuration file,
    <link linkend="auth-username-maps"><filename>pg_ident.conf</filename></link>.
    A row appears in this view for each non-empty, non-comment line in the file,
-   with annotations indicating whether the rule could be applied successfully.
+   with annotations indicating whether the map could be applied successfully.
   </para>
 
   <para>
@@ -1157,7 +1157,7 @@
        <structfield>line_number</structfield> <type>int4</type>
       </para>
       <para>
-       Line number of this rule in <filename>pg_ident.conf</filename>
+       Line number of this map in <filename>pg_ident.conf</filename>
       </para></entry>
      </row>