Fix case of '-f' switch for poetry build (#635)

In version 0.12.8, attempting to use an uppercase 'F' to designate build format yields the error: 

[NoSuchOption]
 The "-F" options does not exist.
 build [-f|--format FORMAT]

Switching to lowercase 'f' to match the CLI's current configuration.

README.md

@@ -443,7 +443,7 @@ Note that, at the moment, only pure python wheels are supported.
 
 #### Options
 
-* `-F|--format`: Limit the format to either wheel or sdist.
+* `-f|--format`: Limit the format to either wheel or sdist.
 
 ### publish