Fix the type of null_value_pattern in configuration/parser-section

`regexp` is appropriate for it.

See also: fluent/fluentd#3650

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

configuration/parse-section.md

@@ -89,7 +89,7 @@ The default value of the following parameters will be overridden by the individu
 
     parameter and set it to `time` by default.
 
-* **`null_value_pattern`** \(string\) \(optional\): Specify null value pattern.
+* **`null_value_pattern`** \(regexp\) \(optional\): Specify null value pattern.
   * Default: `nil`
 * **`null_empty_string`** \(bool\) \(optional\): If `true`, empty string field is