doc: fix table formatting (#51)

Author: jwilk

README.md

@@ -46,8 +46,8 @@ The flags and their restrictions are:
 
 |__Option__     | __Meaning__    | __Type__ | __Default__  |
 | ------------| -------------- | ------------ | ------------ |
-|`-i`  | input file  | ```string | stdin``` | `stdin`
-|`-o`  | output file | ```string | stdout``` |  `stdout` 
+|`-i`  | input file  | `string \| stdin` | `stdin`
+|`-o`  | output file | `string \| stdout` |  `stdout`
 |`-no-digit`  | do not replace variables starting with a digit, e.g. $1 and ${1} | `flag` |  `false` 
 |`-no-unset`  | fail if a variable is not set | `flag` |  `false` 
 |`-no-empty`  | fail if a variable is set but empty | `flag` | `false`