kvs: correct transaction-merge option parsing

Problem: The transaction-merge option was not parsed correctly due
to an invalid index.  This was never uncovered due to previous
invalid input handling.

Correct transaction-merge option parsing.

Author: chu11

src/modules/kvs/kvs.c

@@ -2855,7 +2855,7 @@ static int process_args (struct kvs_ctx *ctx, int ac, char **av)
         if (strstarts (av[i], "transaction-merge=")) {
             char *endptr;
             errno = 0;
-            ctx->transaction_merge = strtoul (av[i]+13, &endptr, 10);
+            ctx->transaction_merge = strtoul (av[i]+18, &endptr, 10);
             if (errno != 0 || *endptr != '\0') {
                 errno = EINVAL;
                 return -1;