Fix man page and usage()

Man page was repeating -f option, the second time instead of -i,
and this option was lost in usage() message. This patch also indent
the output of usage().

st.1

@@ -42,7 +42,7 @@ The form is [=][<cols>{xX}<rows>][{+-}<xoffset>{+-}<yoffset>]. See
 .BR XParseGeometry (3)
 for further details.
 .TP
-.B \-f
+.B \-i
 will fixate the position given with the -g option.
 .TP
 .BI \-o " file"