fix typo pkcs7_depad > padding_depad [skip-ci]

Author: karel-m

doc/crypt.tex

@@ -7204,9 +7204,9 @@ \subsection{Depadding}
 
 \index{padding\_depad()}
 \begin{verbatim}
-int pkcs7_depad(unsigned char *data,
-                unsigned long *length,
-                unsigned long  mode);
+int padding_depad(unsigned char *data,
+                  unsigned long *length,
+                  unsigned long  mode);
 \end{verbatim}
 
 Where \textit{data} is a pointer to the data to depad,