binman: Correct comment in u_boot_spl_elf

This comment mentions the wrong default filename. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README.entries b/tools/binman/README.entries
index 9cbdbba..c26addc 100644
--- a/tools/binman/README.entries
+++ b/tools/binman/README.entries
@@ -496,7 +496,7 @@
 -------------------------------------------
 
 Properties / Entry arguments:
-    - filename: Filename of SPL u-boot (default 'spl/u-boot')
+    - filename: Filename of SPL u-boot (default 'spl/u-boot-spl')
 
 This is the U-Boot SPL ELF image. It does not include a device tree but can
 be relocated to any address for execution.