fix typos

railties/guides/source/routing.textile

@@ -498,7 +498,7 @@ You specify a request-based constraint the same way that you specify a segment c
 match "photos", :constraints => {:subdomain => "admin"}
 </ruby>
 
-You can also specify constrains in a block form:
+You can also specify constraints in a block form:
 
 <ruby>
 namespace :admin do
@@ -598,7 +598,7 @@ You can specify what Rails should route +"/"+ to with the +root+ method:
 root :to => 'pages#main'
 </ruby>
 
-You should put the +root+ route at the end of the file. You also need to delete the public/index.html.erb file for the root route to take effect.
+You should put the +root+ route at the end of the file. You also need to delete the +public/index.html+ file for the root route to take effect.
 
 h3. Customizing Resourceful Routes
 
@@ -633,7 +633,7 @@ You can use the +:constraints+ option to specify a required format on the implic
 resources :photos, :constraints => {:id => /[A-Z][A-Z][0-9]+/}
 </ruby>
 
-This declaration constrains the +:id+ parameter to match the supplied regular expression. So, in this case, the router would no longer match +/photos/1+ to this route. Instead, +/photos/RR27+ would match.
+This declaration constraints the +:id+ parameter to match the supplied regular expression. So, in this case, the router would no longer match +/photos/1+ to this route. Instead, +/photos/RR27+ would match.
 
 You can specify a single constraint to apply to a number of routes by using the block form: