[css-mixins-1] Fix anchor for CSSFunctionDeclarations (#11550)

This section was originally copied from css-nesting-1,
and I must have forgotten to adjust this.

Co-authored-by: Anders Hartvoll Ruud <andruud@google.com>

Author: andruud

css-mixins-1/Overview.bs

@@ -670,8 +670,8 @@ Note: This also applies to the "leading" declarations in the ''@function'' rule,
 </div>
 
 
-The {{CSSFunctionDeclarations}} Interface {#the-cssnestrule}
------------------------------
+The {{CSSFunctionDeclarations}} Interface {#the-function-declarations-interface}
+--------------------------------------------------------------------------------
 
 The {{CSSFunctionDeclarations}} interface represents a run
 of consecutive [=declarations=] within a ''@function'' rule.