correct a typo

guides/source/active_support_core_extensions.textile

@@ -1349,7 +1349,7 @@ The second argument, +indent_string+, specifies which indent string to use. The
 "foo".indent(2, "\t")    # => "\t\tfoo"
 </ruby>
 
-While +indent_string+ is tipically one space or tab, it may be any string.
+While +indent_string+ is typically one space or tab, it may be any string.
 
 The third argument, +indent_empty_lines+, is a flag that says whether empty lines should be indented. Default is false.