Doc: missing word

Either a path or a dictionary whereas the key is the custom index and the value the path.
Either a path or a dictionary whereas the key is the custom index and the value *is* the path.

Closes https://github.com/mhinz/vim-startify/pull/497

doc/startify.txt

@@ -260,7 +260,7 @@ NOTE: Headers use the StartifySection highlight group. See |startify-colors|.
 <
 A list of files or directories to bookmark. The list can contain two kinds of
 types. Either a path or a dictionary whereas the key is the custom index and
-the value the path.
+the value is the path.
 
 Example:
 >