Try to fix doc deployment

https://github.com/harfbuzz/harfbuzz.github.io/issues/4

From CI logs at https://github.com/harfbuzz/harfbuzz/runs/2758234200:

+ git init
hint: Using 'master' as the name for the initial branch. This default branch name
hint: is subject to change. To configure the initial branch name to use in all
hint: of your new repositories, which will suppress this warning, call:
hint:
hint: 	git config --global init.defaultBranch <name>
hint:
hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
hint: 'development'. The just-created branch can be renamed via this command:
hint:
hint: 	git branch -m <name>

We want the branch too be main not master.

.ci/deploy-docs.sh

@@ -16,6 +16,7 @@ cp ../build/docs/html/* .
 #cp ../build/docs/CNAME .
 
 git init
+git branch -m main
 git config user.name "Travis CI"
 git config user.email "travis@harfbuzz.org"
 set +x