Added missing quotation mark in search value.

Keeps overwriting custom path.

I'm running Ubuntu 16.04 and has changed 
x-httpd-suphp="php:/usr/bin/php-cgi"
To
x-httpd-suphp="php:/usr/bin/php-cgi5.6"
And when i update ispconfig it keeps adding this to the end of the file
x-httpd-suphp="php:/usr/bin/php-cgi"

See merge request !358