Update README.md using the correct version - 3.1

Author: renan-braga

mimic-iv/buildmimic/postgres/README.md

@@ -17,13 +17,13 @@ If following the tutorials, be sure to download the scripts locally and the MIMI
 git clone https://github.com/MIT-LCP/mimic-code.git
 cd mimic-code
 # download data
-wget -r -N -c -np --user <USERNAME> --ask-password https://physionet.org/files/mimiciv/2.2/
+wget -r -N -c -np --user <USERNAME> --ask-password https://physionet.org/files/mimiciv/3.1/
 mv physionet.org/files/mimiciv mimiciv && rmdir physionet.org/files && rm physionet.org/robots.txt && rmdir physionet.org
 createdb mimiciv
 psql -d mimiciv -f mimic-iv/buildmimic/postgres/create.sql
-psql -d mimiciv -v ON_ERROR_STOP=1 -v mimic_data_dir=mimiciv/2.2 -f mimic-iv/buildmimic/postgres/load_gz.sql
-psql -d mimiciv -v ON_ERROR_STOP=1 -v mimic_data_dir=mimiciv/2.2 -f mimic-iv/buildmimic/postgres/constraint.sql
-psql -d mimiciv -v ON_ERROR_STOP=1 -v mimic_data_dir=mimiciv/2.2 -f mimic-iv/buildmimic/postgres/index.sql
+psql -d mimiciv -v ON_ERROR_STOP=1 -v mimic_data_dir=mimiciv/3.1 -f mimic-iv/buildmimic/postgres/load_gz.sql
+psql -d mimiciv -v ON_ERROR_STOP=1 -v mimic_data_dir=mimiciv/3.1 -f mimic-iv/buildmimic/postgres/constraint.sql
+psql -d mimiciv -v ON_ERROR_STOP=1 -v mimic_data_dir=mimiciv/3.1 -f mimic-iv/buildmimic/postgres/index.sql
 ```
 
 ## Detailed guide