Cleanup: Missing word in rna description

One of the sentences changed recently in rBc63df3b33f01 was missing
a word. No functional changes.

source/blender/makesrna/intern/rna_space.c

@@ -4801,7 +4801,7 @@ static void rna_def_space_properties(BlenderRNA *brna)
        "AUTO",
        0,
        "Auto",
-       "Change tabs only when this editor shares a border the outliner"},
+       "Change tabs only when this editor shares a border with an outliner"},
       {0, NULL, 0, NULL, NULL},
   };