Fix broken links to plugins page

[griadooss]

Fixed the broken links in both the English and French versions of the Ardour tutorial website:

• The original links were pointing to a non-existent directory path: ../../appendices/plugins/
• The recommended plugins information actually exists in the mixing-sessions section at ../../mixing-sessions/plugins/
• Updated both the English and French versions of the installing-plugins page to point to the correct location