sources.sgml typo

In doc/src/sgml/sources.sgml:

When the definition an inline function references symbols
(i.e. variables, functions) that are only available as part of the
backend, the function may not be visible when included from frontend
code.

Shinichi Matsuda reported that there should be "of" between
"definition" and "an inline function" and I think he is right. Patch
attached. If there's no objection, I will commit it.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp
diff --git a/doc/src/sgml/sources.sgml b/doc/src/sgml/sources.sgml
index d132ee4..877fced 100644
--- a/doc/src/sgml/sources.sgml
+++ b/doc/src/sgml/sources.sgml
@@ -898,7 +898,7 @@ BETTER: unrecognized node type: 42     expressions of various types need to be passed to the macro.
  </para>    <para>
 
-     When the definition an inline function references symbols
+     When the definition of an inline function references symbols     (i.e. variables, functions) that are only
availableas part of the     backend, the function may not be visible when included from frontend     code.