Correct typos that reference a non-existing file

There are references to odb_backends.h when the file is actually named
odb_backend.h and in the sys folder.
diff --git a/include/git2/odb.h b/include/git2/odb.h
index b3ed270..b7dc0c5 100644
--- a/include/git2/odb.h
+++ b/include/git2/odb.h
@@ -488,7 +488,7 @@
  * The backends are checked in relative ordering, based on the
  * value of the `priority` parameter.
  *
- * Read <odb_backends.h> for more information.
+ * Read <sys/odb_backend.h> for more information.
  *
  * @param odb database to add the backend to
  * @param backend pointer to a git_odb_backend instance
@@ -509,7 +509,7 @@
  *
  * Writing is disabled on alternate backends.
  *
- * Read <odb_backends.h> for more information.
+ * Read <sys/odb_backend.h> for more information.
  *
  * @param odb database to add the backend to
  * @param backend pointer to a git_odb_backend instance