apcu-entry.xml Change the parameter name (#4818)

* apcu-entry.xml Change the parameter name

Probably, the <parameter>var</parameter> is an artifact that came from another APCu function, such as `apcu_store()`

* apcu-entry.xml Replace the `variable` to `entry`

Author: mmalferov

reference/apcu/functions/apcu-entry.xml

@@ -57,9 +57,9 @@
      <term><parameter>ttl</parameter></term>
      <listitem>
       <para>
-       Time To Live; store <parameter>var</parameter> in the cache for
+       Time To Live; store the <parameter>callback</parameter>'s return value in the cache for
        <parameter>ttl</parameter> seconds. After the
-       <parameter>ttl</parameter> has passed, the stored variable will be
+       <parameter>ttl</parameter> has passed, the stored entry will be
        expunged from the cache (on the next request). If no <parameter>ttl</parameter>
        is supplied (or if the <parameter>ttl</parameter> is
        <literal>0</literal>), the value will persist until it is removed from