[DOCS] fix broken links to breaking changes docs

Closes elastic#528

Author: polyfractal

BREAKING_CHANGES.md

@@ -3,9 +3,9 @@
 ## Breaking changes
 
 - Indices/Analyze Endpoint: `filters` and `char_filters` URI parameters have renamed to `filter` and `char_filter` respectively
-- SearchExists endpoint has been removed ([use `size=0` and `terminate_after=1` instead](https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking_50_search_changes.html#_search_exists_api_removed))
-- Warmers have been removed because they are [no longer useful](https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking_50_index_apis.html#_warmers)
-- Indices/Optimize Endpoint has been removed ([use `_forcemerge` instead](https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking_50_rest_api_changes.html#_literal__optimize_literal_endpoint_removed))
+- SearchExists endpoint has been removed ([use `size=0` and `terminate_after=1` instead](https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking_50_search_changes.html#_search_exists_api_removed))
+- Warmers have been removed because they are [no longer useful](https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking_50_index_apis.html#_warmers)
+- Indices/Optimize Endpoint has been removed ([use `_forcemerge` instead](https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking_50_rest_api_changes.html#_literal__optimize_literal_endpoint_removed))
 - MoreLikeThis (MLT) endpoint has been removed
 - DeleteByQuery endpoint has been removed.
 - Tasks/List and Tasks/Get are now separate endpoints (see: [[e0cc5f9]](http://github.com/elasticsearch/elasticsearch-php/commit/752d5a2))