fix doc typo

s/it's/its

lib/uri/common.rb

@@ -252,7 +252,7 @@ module URI
   #
   # Returns a Regexp object which matches to URI-like strings.
   # The Regexp object returned by this method includes arbitrary
-  # number of capture group (parentheses).  Never rely on it's number.
+  # number of capture group (parentheses).  Never rely on its number.
   #
   # == Usage
   #