Wrong usage of 'a' in docs fixed [ci skip]

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -458,7 +458,7 @@ def explain(arel, binds = [])
       end
 
       class ExplainPrettyPrinter # :nodoc:
-        # Pretty prints the result of a EXPLAIN in a way that resembles the output of the
+        # Pretty prints the result of an EXPLAIN in a way that resembles the output of the
         # MySQL shell:
         #
         #   +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb

@@ -8,7 +8,7 @@ def explain(arel, binds = [])
         end
 
         class ExplainPrettyPrinter # :nodoc:
-          # Pretty prints the result of a EXPLAIN in a way that resembles the output of the
+          # Pretty prints the result of an EXPLAIN in a way that resembles the output of the
           # PostgreSQL shell:
           #
           #                                     QUERY PLAN

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -218,7 +218,7 @@ def explain(arel, binds = [])
       end
 
       class ExplainPrettyPrinter
-        # Pretty prints the result of a EXPLAIN QUERY PLAN in a way that resembles
+        # Pretty prints the result of an EXPLAIN QUERY PLAN in a way that resembles
         # the output of the SQLite shell:
         #
         #   0|0|0|SEARCH TABLE users USING INTEGER PRIMARY KEY (rowid=?) (~1 rows)

activerecord/lib/active_record/reflection.rb

@@ -62,7 +62,7 @@ def reflect_on_aggregation(aggregation)
         aggregate_reflections[aggregation.to_s]
       end
 
-      # Returns a Hash of name of the reflection as the key and a AssociationReflection as the value.
+      # Returns a Hash of name of the reflection as the key and an AssociationReflection as the value.
       #
       #   Account.reflections # => {"balance" => AggregateReflection}
       #

activesupport/lib/active_support/core_ext/marshal.rb

@@ -6,7 +6,7 @@ def load(source)
       if exc.message.match(%r|undefined class/module (.+)|)
         # try loading the class/module
         $1.constantize
-        # if it is a IO we need to go back to read the object
+        # if it is an IO we need to go back to read the object
         source.rewind if source.respond_to?(:rewind)
         retry
       else