Update docs for -X flag.

-X flag changed subtly with previous pull request. It now requires a
directory name and doesn't work on substrings.

README.md

@@ -41,7 +41,7 @@ Usage: dust -n 30  <dir>  (Shows 30 directories not 20)
 Usage: dust -d 3  <dir>  (Shows 3 levels of subdirectories)
 Usage: dust -r  <dir>  (Reverse order of output, with root at the lowest)
 Usage: dust -x  <dir>  (Only show directories on same filesystem)
-Usage: dust -X ignore  <dir>  (Ignore all files and directories containing the string 'ignore')
+Usage: dust -X ignore  <dir>  (Ignore all files and directories with the name 'ignore')
 ```
 
 ```

src/main.rs

@@ -62,7 +62,7 @@ fn main() {
                 .takes_value(true)
                 .number_of_values(1)
                 .multiple(true)
-                .help("Exclude any file or directory with contains this substring."),
+                .help("Exclude any file or directory with this name."),
         )
         .arg(
             Arg::with_name("limit_filesystem")