fix FFI non-static method is written to static method

Closes GH-1174.

Author: chopins

reference/ffi/ffi/cast.xml

@@ -14,7 +14,7 @@
    <methodparam><type class="union"><type>FFI\CData</type><type>int</type><type>float</type><type>bool</type><type>null</type></type><parameter role="reference">ptr</parameter></methodparam>
   </methodsynopsis>
   <methodsynopsis>
-   <modifier>public</modifier> <modifier>static</modifier> <type class="union"><type>FFI\CData</type><type>null</type></type><methodname>FFI::cast</methodname>
+   <modifier>public</modifier> <type class="union"><type>FFI\CData</type><type>null</type></type><methodname>FFI::cast</methodname>
    <methodparam><type class="union"><type>FFI\CType</type><type>string</type></type><parameter>type</parameter></methodparam>
    <methodparam><type class="union"><type>FFI\CData</type><type>int</type><type>float</type><type>bool</type><type>null</type></type><parameter role="reference">ptr</parameter></methodparam>
   </methodsynopsis>

reference/ffi/ffi/new.xml

@@ -15,7 +15,7 @@
    <methodparam choice="opt"><type>bool</type><parameter>persistent</parameter><initializer>&false;</initializer></methodparam>
   </methodsynopsis>
   <methodsynopsis>
-   <modifier>public</modifier> <modifier>static</modifier> <type class="union"><type>FFI\CData</type><type>null</type></type><methodname>FFI::new</methodname>
+   <modifier>public</modifier> <type class="union"><type>FFI\CData</type><type>null</type></type><methodname>FFI::new</methodname>
    <methodparam><type class="union"><type>FFI\CType</type><type>string</type></type><parameter>type</parameter></methodparam>
    <methodparam choice="opt"><type>bool</type><parameter>owned</parameter><initializer>&true;</initializer></methodparam>
    <methodparam choice="opt"><type>bool</type><parameter>persistent</parameter><initializer>&false;</initializer></methodparam>

reference/ffi/ffi/type.xml

@@ -13,7 +13,7 @@
    <methodparam><type>string</type><parameter>type</parameter></methodparam>
   </methodsynopsis>
   <methodsynopsis>
-   <modifier>public</modifier> <modifier>static</modifier> <type class="union"><type>FFI\CType</type><type>null</type></type><methodname>FFI::type</methodname>
+   <modifier>public</modifier> <type class="union"><type>FFI\CType</type><type>null</type></type><methodname>FFI::type</methodname>
    <methodparam><type>string</type><parameter>type</parameter></methodparam>
   </methodsynopsis>
   <para>