doc: generation: fix link to zephyr.tag

This link probably never worked. This commit fixes the relative
URL to properly point to where Doxygen docs live.

Fixes #78016.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

doc/contribute/documentation/generation.rst

@@ -290,7 +290,7 @@ Linking external Doxygen projects against Zephyr
 
 External projects that build upon Zephyr functionality and wish to refer to
 Zephyr documentation in Doxygen (through the use of @ref), can utilize the
-tag file exported at `zephyr.tag </doxygen/html/zephyr.tag>`_
+tag file exported at `zephyr.tag <../../doxygen/html/zephyr.tag>`_
 
 Once downloaded, the tag file can be used in a custom ``doxyfile.in`` as follows::