Backport PR matplotlib#31990: DOC fix parameter name in CharacterMapping.add docstring

Author: timhoffm

lib/matplotlib/backends/_backend_pdf_ps.py

@@ -149,7 +149,7 @@ def add(self, charcode: str, glyph_index: GlyphIndexType, subset: int,
         ----------
         charcode : CharacterCodeType
             The character code to record.
-        glyph : GlyphIndexType
+        glyph_index : GlyphIndexType
             The corresponding glyph index to record.
         subset : int
             The subset in which the subset character code resides.