[doc] add -H flag to sudo commands

doc/00-intro.md

@@ -109,7 +109,7 @@ mv composer.phar /usr/local/bin/composer
 A quick copy-paste version including sudo:
 
 ```sh
-curl -sS https://getcomposer.org/installer | sudo php -- --install-dir=/usr/local/bin --filename=composer
+curl -sS https://getcomposer.org/installer | sudo -H php -- --install-dir=/usr/local/bin --filename=composer
 ```
 
 > **Note:** On some versions of OSX the `/usr` directory does not exist by

doc/03-cli.md

@@ -413,7 +413,7 @@ If you have installed Composer for your entire system (see [global installation]
 you may have to run the command with `root` privileges
 
 ```sh
-sudo composer self-update
+sudo -H composer self-update
 ```
 
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