docs/manual/migrating.adoc: fix occurrence typo

Occurrence is with double-c-double-r.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 6ee748089347966eea5aced1e42fc25d9cbfc2aa)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/migrating.adoc

@@ -62,7 +62,7 @@ $ echo 'name: NAME_OF_YOUR_TREE' >external.desc
 Be careful when choosing a name: It has to be unique and be made
 with only ASCII characters from the set +[A-Za-z0-9_]+.
 
- * Then, change every occurence of +BR2_EXTERNAL+ in your br2-external
+ * Then, change every occurrence of +BR2_EXTERNAL+ in your br2-external
    tree with the new variable:
 +
 ----