docs: use single quotes for multi-line literal TOML (#13124)

fixes: #13123

plugins/parsers/avro/README.md

@@ -32,7 +32,7 @@ The message is supposed to be encoded as follows:
   avro_schema_registry = "http://localhost:8081"
 
   ## Schema string; exactly one of schema registry and schema must be set
-  #avro_schema = """
+  #avro_schema = '''
   #        {
   #          "type":"record",
   #          "name":"Value",
@@ -52,7 +52,7 @@ The message is supposed to be encoded as follows:
   #              }
   #          ]
   #      }
-  #"""
+  #'''
 
   ## Measurement string; if not set, determine measurement name from
   ## schema (as "<namespace>.<name>")