Fix datasource metrics doc

The maximum and minimum number of connections are no metrics so these
are not exposed. Fix the doc that stated the contrary.

Fixes gh-3319

Author: snicoll

spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -811,8 +811,6 @@ The following system metrics are exposed by Spring Boot:
 The following metrics are exposed for each supported `DataSource` defined in your
 application:
 
-* The maximum number connections (`datasource.xxx.max`).
-* The minimum number of connections (`datasource.xxx.min`).
 * The number of active connections (`datasource.xxx.active`)
 * The current usage of the connection pool (`datasource.xxx.usage`).