README: Fix include file location for plugin

If the CMD_INC_FILE is defined with file name without location, the
following error will happen.

In file included from plugins/foo/foo-nvme.h:17:0,
                 from plugins/foo/foo-nvme.c:4:
./define_cmd.h:12:34: fatal error: foo-nvme.h: No such file or directory
 #include CMD_INCLUDE(CMD_INC_FILE)
                                  ^

Signed-off-by: Minwoo Im <[email protected]>

Author: minwooim

README.md

@@ -176,7 +176,7 @@ is a basic example on how to start this:
 File: foo-plugin.h
 ```c
 #undef CMD_INC_FILE
-#define CMD_INC_FILE foo-plugin
+#define CMD_INC_FILE plugins/foo/foo-plugin
 
 #if !defined(FOO) || defined(CMD_HEADER_MULTI_READ)
 #define FOO