[DOC] Fix typo in docs/globals.rdoc by @dvsuresh [Fixes GH-474]

https://github.com/ruby/ruby/pull/474

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44145 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

doc/globals.rdoc

@@ -42,7 +42,7 @@ $VERBOSE::   The verbose flag, which is set by the -w or -v switch.  Setting
              including from Kernel#warn.
 $-0::  The alias to $/.
 $-a::  True if option -a is set. Read-only variable.
-$-d::  The alias of $DEBUG.  See $DEBUG above for further discusison.
+$-d::  The alias of $DEBUG.  See $DEBUG above for further discussion.
 $-F::  The alias to $;.
 $-i::  In in-place-edit mode, this variable holds the extension, otherwise nil.
 $-I::  The alias to $:.