docs: fix misspellings

Fixed what I noticed; no warranty for exhaustiveness.

Signed-off-by: Nathan Kidd <nkidd@opentext.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

docs/repository.html

@@ -156,7 +156,7 @@ each time you do a pull.
 </p>
 <li>Small changes to master
 <p>
-If you are an experienced git user working on substancial modifications,
+If you are an experienced git user working on substantial modifications,
 you are probably
 working on a separate branch and would rebase your branch prior to
 merging with master.