Fix doc typo in Symbol#to_proc [Fix GH-1785]

[ci skip]

From: Dimitris Zorbas <dimitrisplusplus@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61588 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -10461,7 +10461,7 @@ rb_sym_proc_call(ID mid, int argc, const VALUE *argv, VALUE passed_proc)
  * call-seq:
  *   sym.to_proc
  *
- * Returns a _Proc_ object which respond to the given method by _sym_.
+ * Returns a _Proc_ object which responds to the given method by _sym_.
  *
  *   (1..3).collect(&:to_s)  #=> ["1", "2", "3"]
  */