DOC-2603: Add Preformatted to block_formats (#3567)

Author: Farzad Hayat

modules/ROOT/partials/configuration/block_formats.adoc

@@ -5,7 +5,7 @@ This option defines the formats to be displayed in the `+blocks+` dropdown toolb
 
 *Type:* `+String+`
 
-*Default value:* `+'Paragraph=p; Heading 1=h1; Heading 2=h2; Heading 3=h3; Heading 4=h4; Heading 5=h5; Heading 6=h6;+`
+*Default value:* `+'Paragraph=p; Heading 1=h1; Heading 2=h2; Heading 3=h3; Heading 4=h4; Heading 5=h5; Heading 6=h6; Preformatted=pre'+`
 
 === Example: using `+block_formats+`