Fix misspelling of "substitution".

PR:		bin/130874
Submitted by:	Yoshihiro Ota

contrib/nvi/docs/help

@@ -129,7 +129,7 @@ EX COMMANDS:
 	  ^D: scroll lines
 	   !: filter lines through commands or run commands
 	   #: display numbered lines
-	   &: repeat the last subsitution
+	   &: repeat the last substitution
 	   *: execute a buffer
 	   <: shift lines left
 	   =: display line number

contrib/nvi/ex/ex_cmd.c

@@ -68,7 +68,7 @@ EXCMDLIST const cmds[] = {
 	{"&",		ex_subagain,	E_ADDR2,
 	    "s",
 	    "[line [,line]] & [cgr] [count] [#lp]",
-	    "repeat the last subsitution"},
+	    "repeat the last substitution"},
 /* C_STAR */
 	{"*",		ex_at,		0,
 	    "b",