IR: fix some doxygen grammar (NFC)

Fix a bit of the doxygen grammar that was off that I noticed while
looking at this file for another issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301658 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DebugInfoMetadata.h

@@ -435,7 +435,7 @@ class DIScope : public DINode {
 
   /// Return the raw underlying file.
   ///
-  /// An \a DIFile is an \a DIScope, but it doesn't point at a separate file
+  /// A \a DIFile is a \a DIScope, but it doesn't point at a separate file
   /// (it\em is the file).  If \c this is an \a DIFile, we need to return \c
   /// this.  Otherwise, return the first operand, which is where all other
   /// subclasses store their file pointer.