Improve PostgreSQL plugin documentation (#2777)

plugins/inputs/postgresql_extensible/README.md

@@ -130,9 +130,9 @@ create extension pg_proctab;
  - pg_stat_kcache is available on the postgresql.org yum repo
  - pg_proctab is available at : https://github.com/markwkm/pg_proctab
 
- ##Views
+ ## Views
  - Blocking sessions
-```
+```sql
 CREATE OR REPLACE VIEW public.blocking_procs AS
  SELECT a.datname AS db,
     kl.pid AS blocking_pid,
@@ -156,7 +156,7 @@ CREATE OR REPLACE VIEW public.blocking_procs AS
   ORDER BY a.query_start;
 ```
   - Sessions Statistics
-```
+```sql
 CREATE OR REPLACE VIEW public.sessions AS
  WITH proctab AS (
          SELECT pg_proctab.pid,