Fix typo in Makefile for installcheck-resgroup target

The --load-extension option was misspelt as load_extension.  Resource group
tests are difficult to run locally, neither are they run in PR pipeline.  That
makes it difficult to catch such errors.

src/test/isolation2/Makefile

@@ -59,4 +59,4 @@ installcheck: install
 	./pg_isolation2_regress $(EXTRA_REGRESS_OPTS) --init-file=$(top_builddir)/src/test/regress/init_file --init-file=./init_file_isolation2 --psqldir='$(PSQLDIR)' --inputdir=$(srcdir) --ao-dir=uao --load-extension=gp_inject_fault --schedule=$(srcdir)/isolation2_schedule
 
 installcheck-resgroup: install
-	./pg_isolation2_regress $(EXTRA_REGRESS_OPTS) --init-file=$(top_builddir)/src/test/regress/init_file --init-file=./init_file_resgroup --psqldir='$(PSQLDIR)' --inputdir=$(srcdir) --dbname=isolation2resgrouptest --load_extension=gp_inject_fault --schedule=$(srcdir)/isolation2_resgroup_schedule
+	./pg_isolation2_regress $(EXTRA_REGRESS_OPTS) --init-file=$(top_builddir)/src/test/regress/init_file --init-file=./init_file_resgroup --psqldir='$(PSQLDIR)' --inputdir=$(srcdir) --dbname=isolation2resgrouptest --load-extension=gp_inject_fault --schedule=$(srcdir)/isolation2_resgroup_schedule