add syntax highlight for lua (#179)

manual/package_dependencies.md

@@ -181,7 +181,7 @@ add_links("mbedtls", "mbedx509", "mbedcrypto")
 
 Add some system library links. When some packages integrate links, you also need to rely on some system libraries to link them. This time you can attach them to the package description.
 
-```
+```lua
 if is_plat("macosx") then
     add_frameworks("CoreGraphics", "CoreFoundation", "Foundation")
 elseif is_plat("windows") then

zh-cn/manual/package_dependencies.md

@@ -178,7 +178,7 @@ add_links("mbedtls", "mbedx509", "mbedcrypto")
 
 添加一些系统库链接，有些包集成链接的时候，还需要依赖一些系统库，才能链接通过，这个时候可以在包描述里面都附加上去。
 
-```
+```lua
 if is_plat("macosx") then
     add_frameworks("CoreGraphics", "CoreFoundation", "Foundation")
 elseif is_plat("windows") then