docs: fix typos and use better wording

Author: evanlinjin

README.md

@@ -249,7 +249,7 @@ assert_eq!(selected_coins.len(), 1);
 let drain = selector.drain(target, change_policy);
 
 if drain.is_some() {
-    // add our change outupt to the transaction
+    // add our change output to the transaction
     let change_value = drain.value;
 }
 ```

src/coin_selector.rs

@@ -666,7 +666,7 @@ impl DrainWeights {
             + self.spend_weight as f32 * long_term_feerate.spwu()
     }
 
-    /// The the fee you will pay to spend this otuput in the future.
+    /// The fee you will pay to spend these change output(s) in the future.
     pub fn spend_fee(&self, long_term_feerate: FeeRate) -> u64 {
         (self.spend_weight as f32 * long_term_feerate.spwu()).ceil() as u64
     }