Formatting Fix. Changed " to `` around the word 'constant' in the Lang Ref
section Global Variable so that the style matches the other keywords in
said section.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174040 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed a mistake in my previous commit where I changed the wording slightly and forgot to undo the change after changing my mind and deciding to only commit the style changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174041 91177308-0d34-0410-b5e6-96231b3b80d8