Fix a typo in a spec description

I was perusing some old commits that @onsi had worked on, because, you know,
sometimes you gotta do that, and I found this test had been committed WITH a typo.
Fast forward to current git history, dang, that typo is still there.

Nothing better than fixing typos that have been in your source code for years upon years.

Signed-off-by: Rachel Brindle <rbrindle@pivotal.io>

Foundation/Spec/Extensions/NSObjectSpec+MethodRedirection.mm

@@ -65,7 +65,7 @@ - (NSString *)stodgify:(NSString *)string
             [Redirectable redirectSelector:@selector(cheekify:) to:@selector(cheekify_original:) andRenameItTo:@selector(cheekify_other:)];
         });
 
-        it(@"should do nothing when the new selector name (i.e. the argumen to andRenameItTo:) already exists", ^{
+        it(@"should do nothing when the new selector name (i.e. the argument to andRenameItTo:) already exists", ^{
             [Redirectable redirectSelector:@selector(cheekify:) to:@selector(cheekify_new:) andRenameItTo:@selector(stodgify:)];
 
             [redirectable stodgify:@"Herman"] should equal(@"Herman is so stodgy");