binman: Correct '-a' description in the doc

It needs a space around '-a'.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/tools/binman/binman.rst b/tools/binman/binman.rst
index 1aa2459..b3df3a6 100644
--- a/tools/binman/binman.rst
+++ b/tools/binman/binman.rst
@@ -322,9 +322,9 @@
 command line. For example some entries need access to files and it is not
 always convenient to put these filenames in the image definition (device tree).
 
-The-a option supports this::
+The -a option supports this::
 
-    -a<prop>=<value>
+    -a <prop>=<value>
 
 where::