Fix doc for Hash#dig and Struct#dig

* hash.c (rb_hash_dig): [DOC] correct argument name
  in method description; fix formatting in examples.
* struct.c (rb_struct_dig): ditto.

[ruby-core:79221] [Bug #13148]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57735 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hash.c

@@ -2862,7 +2862,7 @@ rb_hash_any_p(VALUE hash)
  * call-seq:
  *   hsh.dig(key, ...)                 -> object
  *
- * Extracts the nested value specified by the sequence of <i>idx</i>
+ * Extracts the nested value specified by the sequence of <i>key</i>
  * objects by calling +dig+ at each step, returning +nil+ if any
  * intermediate step is +nil+.
  *

struct.c

@@ -1130,7 +1130,7 @@ rb_struct_size(VALUE s)
  * call-seq:
  *   struct.dig(key, ...)              -> object
  *
- * Extracts the nested value specified by the sequence of <i>idx</i>
+ * Extracts the nested value specified by the sequence of +key+
  * objects by calling +dig+ at each step, returning +nil+ if any
  * intermediate step is +nil+.
  *