A minor markup fix:

- `to' isn't a variable
  - don't omit the first `E' in `ERANGE', even though .Er
    is able to take care of it

lib/libm/common_source/exp.3

@@ -150,8 +150,9 @@ and
 .Fn pow
 detect if the computed value will overflow,
 set the global variable
-.Va errno to
-.Er RANGE
+.Va errno
+to
+.Er ERANGE
 and cause a reserved operand fault on a
 .Tn VAX
 or