docs: added a missing export into the plan stability section (#2071)

Author: akupchinskiy

docs/source/contributor-guide/development.md

@@ -108,15 +108,15 @@ Note that the output files get written to `$SPARK_HOME`.
 The tests can be run with:
 
 ```sh
-export SPARK_HOME=`pwd`
+export SPARK_HOME=`pwd` COMET_PARQUET_SCAN_IMPL=native_comet
 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV1_4_PlanStabilitySuite" -Pspark-3.4 -nsu test
 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV1_4_PlanStabilitySuite" -Pspark-3.5 -nsu test
 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV1_4_PlanStabilitySuite" -Pspark-4.0 -nsu test
 ```
 
 and
 ```sh
-export SPARK_HOME=`pwd`
+export SPARK_HOME=`pwd` COMET_PARQUET_SCAN_IMPL=native_comet
 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV2_7_PlanStabilitySuite" -Pspark-3.4 -nsu test
 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV2_7_PlanStabilitySuite" -Pspark-3.5 -nsu test
 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV2_7_PlanStabilitySuite" -Pspark-4.0 -nsu test
@@ -126,15 +126,15 @@ If your pull request changes the query plans generated by Comet, you should rege
 To regenerate the golden files, you can run the following commands.
 
 ```sh
-export SPARK_HOME=`pwd`
+export SPARK_HOME=`pwd` COMET_PARQUET_SCAN_IMPL=native_comet
 SPARK_GENERATE_GOLDEN_FILES=1 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV1_4_PlanStabilitySuite" -Pspark-3.4 -nsu test
 SPARK_GENERATE_GOLDEN_FILES=1 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV1_4_PlanStabilitySuite" -Pspark-3.5 -nsu test
 SPARK_GENERATE_GOLDEN_FILES=1 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV1_4_PlanStabilitySuite" -Pspark-4.0 -nsu test
 ```
 
 and
 ```sh
-export SPARK_HOME=`pwd`
+export SPARK_HOME=`pwd` COMET_PARQUET_SCAN_IMPL=native_comet
 SPARK_GENERATE_GOLDEN_FILES=1 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV2_7_PlanStabilitySuite" -nsu test
 SPARK_GENERATE_GOLDEN_FILES=1 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV2_7_PlanStabilitySuite" -Pspark-3.5 -nsu test
 SPARK_GENERATE_GOLDEN_FILES=1 ./mvnw -pl spark -Dsuites="org.apache.spark.sql.comet.CometTPCDSV2_7_PlanStabilitySuite" -Pspark-4.0 -nsu test