cmake: Fix typo in doc file

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

doc/CMakeLists.txt

@@ -13,7 +13,7 @@ if (DOXYGEN_FOUND)
     set(DOXYGEN_MARKDOWN_SUPPORT YES)
 
     set(DOXYGEN_PREDEFINED DOXYGEN
-                           PRINTF_ATTRIBUTE(x,y)=)
+                           PRINTF_ATTRIBUTE(x,y))
 
     set(DOXYGEN_EXCLUDE ${CMAKE_CURRENT_SOURCE_DIR}/that_style)
     set(DOXYGEN_HTML_HEADER ${CMAKE_CURRENT_SOURCE_DIR}/that_style/header.html)