fix: correct capitalization

doc/usage/referencing.rst

@@ -254,7 +254,7 @@ The following role creates a cross-reference to a term in a
    If you use a term that's not explained in a glossary, you'll get a warning
    during build.
 
-   This role also supports :ref:`Custom link text <xref-modifiers>` from the general
+   This role also supports :ref:`custom link text <xref-modifiers>` from the general
    cross-reference syntax.