[DOCS] Fixing formatting issues in breaking changes

Author: lcawl

docs/reference/migration/migrate_7_0/api.asciidoc

@@ -1,5 +1,5 @@
 [[breaking_70_api_changes]]
-=== Breaking API changes in 7.0
+=== API changes
 
 ==== Camel case and underscore parameters deprecated in 6.x have been removed
 A number of duplicate parameters deprecated in 6.x have been removed from

docs/reference/migration/migrate_7_0/java.asciidoc

@@ -13,7 +13,7 @@ The `prepareExecute` method which created a request builder has been
 removed from the client api. Instead, construct a builder for the
 appropriate request directly.
 
-=== Some Aggregation classes have moved packages
+==== Some Aggregation classes have moved packages
 
 * All classes present in `org.elasticsearch.search.aggregations.metrics.*` packages
 were moved to a single `org.elasticsearch.search.aggregations.metrics` package.

docs/reference/migration/migrate_7_0/mappings.asciidoc

@@ -31,15 +31,15 @@ the index setting `index.mapping.nested_objects.limit`.
 
 This option is useless now that all indices have at most one type.
 
-=== The `classic` similarity has been removed
+==== The `classic` similarity has been removed
 
 The `classic` similarity relied on coordination factors for scoring to be good
 in presence of stopwords in the query. This feature has been removed from
 Lucene, which means that the `classic` similarity now produces scores of lower
 quality. It is advised to switch to `BM25` instead, which is widely accepted
 as a better alternative.
 
-=== Similarities fail when unsupported options are provided
+==== Similarities fail when unsupported options are provided
 
 An error will now be thrown when unknown configuration options are provided
 to similarities. Such unknown parameters were ignored before.