OSSL_PARAM example code bug fix.

Technically not a bug since the code worked but the array index shouldn't have
been constant after searching for the field.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9136)

doc/man3/OSSL_PARAM.pod

@@ -276,8 +276,8 @@ could fill in the parameters like this:
             *(char **)params[i].data = "foo value";
             *params[i].return_size = 10; /* size of "foo value" */
         } else if (strcmp(params[i].key, "bar") == 0) {
-            memcpy(params[1].data, "bar value", 10);
-            *params[1].return_size = 10; /* size of "bar value" */
+            memcpy(params[i].data, "bar value", 10);
+            *params[i].return_size = 10; /* size of "bar value" */
         }
         /* Ignore stuff we don't know */
     }