Fix type of 2nd parameter of openssl_csr_export()

Cf. <php/php-src#8362>.

Author: cmb69

reference/openssl/functions/openssl-csr-export.xml

@@ -11,7 +11,7 @@
   <methodsynopsis>
    <type>bool</type><methodname>openssl_csr_export</methodname>
    <methodparam><type class="union"><type>OpenSSLCertificateSigningRequest</type><type>string</type></type><parameter>csr</parameter></methodparam>
-   <methodparam><type>OpenSSLAsymmetricKey</type><parameter role="reference">output</parameter></methodparam>
+   <methodparam><type>string</type><parameter role="reference">output</parameter></methodparam>
    <methodparam choice="opt"><type>bool</type><parameter>no_text</parameter><initializer>&true;</initializer></methodparam>
   </methodsynopsis>
   <para>