[DOCS] output_mode should be output_type

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/logicaldecoding-output-plugin.html
Description:

Hello.

I found documentation bug.
[47.6.3. Output Modes]
https://www.postgresql.org/docs/9.6/static/logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-MODE

According to this document, "output_mode" parameter is used for changing
output mode. But test_decoding uses "output_type" like below.

--- test_decoding.c ---
 155                 opt->output_type = OUTPUT_PLUGIN_BINARY_OUTPUT;
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Sorry for trivial comment, but please fix this!