fix two typos: and array -> an array

railties/guides/source/active_support_core_extensions.textile

@@ -2314,7 +2314,7 @@ NOTE: Defined in +active_support/core_ext/array/grouping.rb+.
 
 h5. +in_groups(number, fill_with = nil)+
 
-The method +in_groups+ splits an array into a certain number of groups. The method returns and array with the groups:
+The method +in_groups+ splits an array into a certain number of groups. The method returns an array with the groups:
 
 <ruby>
 %w(1 2 3 4 5 6 7).in_groups(3)
@@ -2746,7 +2746,7 @@ Active Support extends the method +Range#step+ so that it can be invoked without
 (1..10).step(2) # => [1, 3, 5, 7, 9]
 </ruby>
 
-As the example shows, in that case the method returns and array with the corresponding elements.
+As the example shows, in that case the method returns an array with the corresponding elements.
 
 NOTE: Defined in +active_support/core_ext/range/blockless_step.rb+.