docs/devel/writing-qmp-commands.txt: Fix docs

Fix the example of add qmp hello-world example.
Without ":", make will report error:
../qapi/misc.json:573:2: line should end with ':'

Signed-off-by: Zihao Chang <changzihao1@huawei.com>
Message-Id: <20201201143308.1626-1-changzihao1@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/devel/writing-qmp-commands.txt

@@ -243,7 +243,7 @@ There are many examples of such documentation in the schema file already, but
 here goes "hello-world"'s new entry for qapi/misc.json:
 
 ##
-# @hello-world
+# @hello-world:
 #
 # Print a client provided string to the standard output stream.
 #