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_docmd.c b/src/ex_docmd.c
index 502f33e..85d4df6 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -7343,7 +7343,7 @@
     int		magic_save = p_magic;
 
     p_magic = (eap->cmdidx == CMD_smagic);
-    do_sub(eap);
+    ex_substitute(eap);
     p_magic = magic_save;
 }