Use common Rust naming convention for Target enum

This is would be a breaking change if it was in the library side of
things but it's in the binary side (`main.rs`) so it shouldn't be
a breaking change.

src/main.rs

@@ -26,8 +26,8 @@ struct Args {
 
 enum Target {
     // NATIONAL,
-    POSTCODE,
-    REGION,
+    Postcode,
+    Region,
 }
 
 impl FromStr for Target {
@@ -36,8 +36,8 @@ impl FromStr for Target {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
             //"" => Ok(Target::NATIONAL),
-            _ if s.parse::<u8>().is_ok() => Ok(Target::REGION),
-            _ => Ok(Target::POSTCODE),
+            _ if s.parse::<u8>().is_ok() => Ok(Target::Region),
+            _ => Ok(Target::Postcode),
         }
     }
 }
@@ -54,25 +54,25 @@ async fn main() {
         let end_date: Option<&str> = args.end_date.as_deref();
 
         match target {
-            Target::POSTCODE => {
+            Target::Postcode => {
                 let result =
                     get_intensities_postcode(args.value.as_str(), start_date, &end_date).await;
                 handle_results(result);
             }
-            Target::REGION => {
+            Target::Region => {
                 let id: u8 = args.value.parse::<u8>().unwrap();
                 let result = get_intensities_region(id, start_date, &end_date).await;
                 handle_results(result);
             }
         }
     } else {
         match target {
-            Target::POSTCODE => {
+            Target::Postcode => {
                 let postcode = args.value.as_str();
                 let result = get_intensity_postcode(postcode).await;
                 handle_result(result, "postcode", postcode);
             }
-            Target::REGION => {
+            Target::Region => {
                 let id: u8 = args.value.parse::<u8>().unwrap();
                 let result = get_intensity_region(id).await;
                 handle_result(result, "region", id.to_string().as_str());