[DOCS] Fine-tune code snippet.

Author: szabosteve

docs/api-conventions.asciidoc

@@ -157,8 +157,8 @@ features guide you to build and navigate complex nested structures:
  use the same conventions as regular properties and accept both a builder lambda 
  expression and a ready-made object of the actual type of the variant. Here’s an 
  example to build a term query:
- +
- --
++
+--
 ["source","java"]
 --------------------------------------------------
 Query query = new Query.Builder()
@@ -171,7 +171,7 @@ Query query = new Query.Builder()
 <1> Choose the `term` variant to build a term query.
 <2> Build the terms query with a builder lambda expression.
 <3> Build the `Query` that now holds a `TermQuery` object with tag `term`.
- --
+--
 
 * Variant objects have getter methods for every available implementation. These 
  methods check that the object actually holds a variant of that type and return