Update branch name to main in env_librados.md (#8738)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/8738

Reviewed By: ltamasi

Differential Revision: D30705691

Pulled By: akankshamahajan15

fbshipit-source-id: 44ac8e1c906b1d1d31e9017a700aab5eefe94253

utilities/env_librados.md

@@ -40,7 +40,7 @@ Status s = DB::Open(options, kDBPath, &db);
 
 # Performance Test
 ## Compile
-Check this [link](https://github.com/facebook/rocksdb/blob/master/INSTALL.md) to install the dependencies of RocksDB. Then you can compile it by running `$ make env_librados_test ROCKSDB_USE_LIBRADOS=1` under `rocksdb\`. The configure file used by env_librados_test is `../ceph/src/ceph.conf`. For Ubuntu 14.04, just run following commands:
+Check this [link](https://github.com/facebook/rocksdb/blob/main/INSTALL.md) to install the dependencies of RocksDB. Then you can compile it by running `$ make env_librados_test ROCKSDB_USE_LIBRADOS=1` under `rocksdb\`. The configure file used by env_librados_test is `../ceph/src/ceph.conf`. For Ubuntu 14.04, just run following commands:
 ```bash
 $ sudo apt-get install libgflags-dev
 $ sudo apt-get install libsnappy-dev