[chore] fix typo in ballast extension's README (#7431)

extension/ballastextension/README.md

@@ -23,7 +23,7 @@ When `size_in_percentage` is enabled with the value(1-100), the absolute `ballas
 1. Look up Memory Cgroup subsystem on the target host or container, find out if there is any total memory limitation has been set for the running collector process.
    Check the value in `memory.limit_in_bytes` file under cgroup memory files (eg, `/sys/fs/cgroup/memory/memory.limit_in_bytes`).
 
-2. If `memory.limit_in_bytes` is positive value other than `9223372036854771712`(`0x7FFFFFFFFFFFF000`). The `ballest_size`
+2. If `memory.limit_in_bytes` is positive value other than `9223372036854771712`(`0x7FFFFFFFFFFFF000`). The `ballast_size`
    will be calculated by `memory.limit_in_bytes * size_in_percentage / 100`.
    If `memory.limit_in_bytes` value is `9223372036854771712`(`0x7FFFFFFFFFFFF000`), it indicates there is no memory limit has
    been set for the collector process or the running container in cgroup. Then the `totalMemory` will be determined in next step.