Fix rdoc syntax for errors#add (rails#46857)

Author: zzak

activemodel/lib/active_model/error.rb

@@ -121,9 +121,10 @@ def initialize_dup(other) # :nodoc:
     attr_reader :attribute
     # The type of error, defaults to +:invalid+ unless specified
     attr_reader :type
-    # The raw value provided as the second parameter when calling +errors#add+
+    # The raw value provided as the second parameter when calling
+    # <tt>errors#add</tt>
     attr_reader :raw_type
-    # The options provided when calling +errors#add+
+    # The options provided when calling <tt>errors#add</tt>
     attr_reader :options
 
     # Returns the error message.