[DOCS] replace '+' with +

Author: javanna

docs/reference/migration/migrate_6_0/indices.asciidoc

@@ -63,8 +63,8 @@ The index parameter in the delete index API no longer accepts alias names.
 Instead, it accepts only index names (or wildcards which will expand to
 matching indices).
 
-==== Support for '+' has been removed in index expressions
+==== Support for `+` has been removed in index expressions
 
-Omitting the '+' has the same effect as specifying it, hence support for '+'
+Omitting the `+` has the same effect as specifying it, hence support for `+`
 has been removed in index expressions.