Hash#fetch: fix grammar in documentation.

[Fix GH-1515][ci skip]
Author:    Alyssa Ross <hi+services.github@alyssa.is>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57390 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hash.c

@@ -876,9 +876,9 @@ rb_hash_lookup(VALUE hash, VALUE key)
  *
  *  Returns a value from the hash for the given key. If the key can't be
  *  found, there are several options: With no other arguments, it will
- *  raise an <code>KeyError</code> exception; if <i>default</i> is
- *  given, then that will be returned; if the optional code block is
- *  specified, then that will be run and its result returned.
+ *  raise a <code>KeyError</code> exception; if <i>default</i> is given,
+ *  then that will be returned; if the optional code block is specified,
+ *  then that will be run and its result returned.
  *
  *     h = { "a" => 100, "b" => 200 }
  *     h.fetch("a")                            #=> 100