docs/manual: fix "+size = +" latex formatting

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 46beca0656505e9a9d4d5d17d22f40550ac487c6)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/writing-rules.txt

@@ -184,7 +184,7 @@ image sdimage.img {
 * Every node(+section+, +partition+, +file+, +subnode+) must have an open
   curly bracket on the same line of the node's name, while the closing one
   must be on a newline and after it a newline must be added except for the
-  last one node. Same goes for its option, for example option +size = +.
+  last one node. Same goes for its option, for example option +size+ +=+.
 
 * Every +option+(i.e. +image+, +offset+, +size+) must have the +=+
   assignment one space from it and one space from the value specified.