examples/elf: Fix invalid preprocessor directive syntax

Fix invalid preprocessor directive in elf_main.c by changing
`#  Warning "No file system selected"` to `#  warning "No file system selected"`.

The incorrect uppercase "Warning" directive would cause compilation
errors, while the proper lowercase "warning" directive generates
the intended compiler warning when no file system is selected.

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

Author: no1wudi

examples/elf/elf_main.c

@@ -313,7 +313,7 @@ int main(int argc, FAR char *argv[])
     }
 #endif
 #else
-#  Warning "No file system selected"
+#  warning "No file system selected"
 #endif
 
   mm_update(&g_mmstep, "after mount");