Fix a minor bug in the regression script during assigning value (#11215)

Summary:
Same as title

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11215

Test Plan: Ran manually

Reviewed By: pdillinger

Differential Revision: D43194634

Pulled By: akankshamahajan15

fbshipit-source-id: 336a08a9076b222d7000e4eb2a87fc36b863b05b

tools/regression_test.sh

@@ -242,7 +242,7 @@ function run_db_bench {
 
   seek_nexts=$SEEK_NEXTS
   if [ "$async_io" == "true" ]; then
-    $seek_nexts=$SEEK_NEXTS_ASYNCIO
+    seek_nexts=$SEEK_NEXTS_ASYNCIO
   fi