binman: Correct an error in the FIT-template example

The example contains references to an non-existent node. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/binman.rst b/tools/binman/binman.rst
index 872e974..0cafc36 100644
--- a/tools/binman/binman.rst
+++ b/tools/binman/binman.rst
@@ -1211,7 +1211,7 @@
 
         spi-image {
             filename = "image-spi.bin";
-            insert-template = <&fit>;
+            insert-template = <&common_part>;
 
             /* things specific to SPI follow */
             footer {
@@ -1224,7 +1224,7 @@
 
         mmc-image {
             filename = "image-mmc.bin";
-            insert-template = <&fit>;
+            insert-template = <&common_part>;
 
             /* things specific to MMC follow */
             footer {