src/sage_docbuild/conf.py: simplify path to tex-chtml.js (mathjax)

Use "tex-chtml.js" instead of "mathjax/tex-chtml.js" as sphinx's
mathjax_path, relative to MATHJAX_DIR. The additional "mathjax"
subdirectory will be added to MATHJAX_DIR by spkg-configure and
sage_conf.

Closes: sagemath#30296

src/sage_docbuild/conf.py

@@ -374,7 +374,7 @@ def set_intersphinx_mappings(app, config):
 if os.environ.get('SAGE_USE_CDNS', 'no') == 'yes':
     mathjax_path = "https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js"
 else:
-    mathjax_path = 'mathjax/tex-chtml.js'
+    mathjax_path = 'tex-chtml.js'
     html_common_static_path += [MATHJAX_DIR]
 
 # A list of glob-style patterns that should be excluded when looking for source