Fix test name typo in PlainTableDBTest

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/4926

Differential Revision: D13830196

Pulled By: siying

fbshipit-source-id: e06bf2a6cd273b5eb18dfd82bdd35ffce197d021

db/plain_table_db_test.cc

@@ -555,7 +555,7 @@ TEST_P(PlainTableDBTest, Flush2) {
   }
 }
 
-TEST_P(PlainTableDBTest, Immortable) {
+TEST_P(PlainTableDBTest, Immortal) {
   for (EncodingType encoding_type : {kPlain, kPrefix}) {
     Options options = CurrentOptions();
     options.create_if_missing = true;