patch 8.2.1436: function implementing :substitute has unexpected name

Problem:    Function implementing :substitute has unexpected name.
Solution:   Rename from do_sub() to ex_substitute().
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 88161fa..2550f0e 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3553,7 +3553,7 @@
  * The usual escapes are supported as described in the regexp docs.
  */
     void
-do_sub(exarg_T *eap)
+ex_substitute(exarg_T *eap)
 {
     linenr_T	lnum;
     long	i = 0;