errdef.h: clarify ERR_VWOEQ

Most often, the reason for this error is a misspelled keyword, not
a missing equals sign in a variable assignment, so we hint at that.

ttbasic/errdef.h

@@ -18,7 +18,7 @@ ESTR(ERR_IFSTKOF, "IF stack overflow")
 ESTR(ERR_IFSTKUF, "IF stack underflow")
 ESTR(ERR_ULN, "Undefined line number")
 ESTR(ERR_PAREN, "\'(\' or \')\' expected")
-ESTR(ERR_VWOEQ, "\'=\' expected")
+ESTR(ERR_VWOEQ, "\'=\' expected (misspelled command?)")
 ESTR(ERR_COM, "Cannot use system command") // v0.83 メッセージ文変更
 ESTR(ERR_VALUE, "Illegal value")      // 追加
 ESTR(ERR_RANGE, "Out of range value") // 追加