Fix spelling: occurences -> occurrences,

[florianjacob]

was found in comments and code, but required no user-facing API change.
Tests were execute successfully.

The misspelled comments lead me to a

#[structopt(short = "v", long = "verbose", parse(from_occurences))]

which fails with a

error: proc-macro derive panicked                                                                                                           
  --> src/main.rs:17:10                                                                                                                     
   |                                                                                                                                        
17 | #[derive(StructOpt, Debug)]                                                                                                            
   |          ^^^^^^^^^                                                                                                                     
   |                                                                                                                                        
   = help: message: called `Result::unwrap()` on an `Err` value: "unsupported parser from_occurences"                                       
                                                                                                                                            
error: aborting due to previous error

[TeXitoi]

Thanks!