Fixes issue typo on MySQL performance

thanks to @shakuzen for noticing

README.md

@@ -65,7 +65,7 @@ storage.close();
 The [InMemoryStorage](zipkin/src/main/java/zipkin/storage/InMemoryStorage.java) component is packaged in zipkin's core library. It is not persistent, nor viable for realistic work loads. Its purpose is for testing, for example starting a server on your laptop without any database needed.
 
 ### MySQL
-The [MySQLStorage](zipkin-storage/mysql) component currently is only tested with MySQL 5.6-7. It is designed to be easy to understand, and get started with. For example, it deconstructs spans into columns, so you can perform ad-hoc queries using SQL. However, this component has [known performance issues](https://github.com/openzipkin/zipkin/issues/233): queries will eventually take seconds to return if you put a lot of data into it.
+The [MySQLStorage](zipkin-storage/mysql) component currently is only tested with MySQL 5.6-7. It is designed to be easy to understand, and get started with. For example, it deconstructs spans into columns, so you can perform ad-hoc queries using SQL. However, this component has [known performance issues](https://github.com/openzipkin/zipkin/issues/1233): queries will eventually take seconds to return if you put a lot of data into it.
 
 ### Cassandra
 The [CassandraStorage](zipkin-storage/cassandra) component is tested against Cassandra 2.2+. It stores spans as opaque thrifts which means you can't read them in cqlsh. However, it is designed for scale. For example, it has manually implemented indexes to make querying larger data more performant. This store requires a [spark job](https://github.com/openzipkin/zipkin-dependencies) to aggregate dependency links.