[Bug] Fix TypeError bug

[frankstorming]

Motivation

I tried to run this command to convert my rctw format dataset to the standard format.
python tools/dataset_converters/textrecog/rctw_converter.py PATH/TO/rctw --nproc 4

Modification

I traced the bug and found the out_json_name was not correct.
In 'mmocr/tools/dataset_converters/textrecog/rctw_converter.py', I found variable dst_label_file was not correct.
I replaced the string 'format' into 'json' in both line 157 and line 159 then the command work!

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After PR:

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[gaotongxiao]

For some reason, the PR you submitted had changed the last characters of dict, and I helped you revert back those unnecessary changes. Thanks!