Fix API docs CI by asking composer his bin dir path

See: https://stackoverflow.com/a/64545124/5155484
See: https://travis-ci.org/github/phpmyadmin/sql-parser/jobs/742510993#L490
Signed-off-by: William Desportes <[email protected]>

Author: williamdes

.travis.yml

@@ -38,7 +38,7 @@ matrix:
         - composer global require "code-lts/doctum:^5.0"
         - composer install --no-interaction
       script:
-        - $HOME/.composer/vendor/bin/doctum.php --no-interaction update ./tools/doctum-config.php
+        - $(composer global config bin-dir --absolute --quiet)/doctum.php --no-interaction update ./tools/doctum-config.php
     - php: nightly
       install:
         - composer install --no-interaction --ignore-platform-reqs