Use monotype font.

So it’s more in line with the rest of the doc.

Author: Renke Christian von Seggern

src/main/antora/modules/ROOT/pages/mongodb/mapping/mapping.adoc

@@ -63,7 +63,7 @@ The following outlines what type conversion, if any, will be done on the propert
 * If a field named `id` is declared as a String or BigInteger in the Java class it will be converted to and stored as an ObjectId if possible.
 ObjectId as a field type is also valid.
 If you specify a value for `id` in your application, the conversion to an ObjectId is done by the MongoDB driver.
-If the specified `id` value cannot be converted to an ObjectId, then the value will be stored as is in the document's _id field.
+If the specified `id` value cannot be converted to an ObjectId, then the value will be stored as is in the document's `_id` field.
 This also applies if the field is annotated with `@Id`.
 * If a field is annotated with `@MongoId` in the Java class it will be converted to and stored as using its actual type.
 No further conversion happens unless `@MongoId` declares a desired field type.