small readme cleanup

README.md

@@ -42,12 +42,9 @@ extern crate rocksdb;
 use rocksdb::{RocksDBOptions, RocksDB, MergeOperands};
 
 fn concat_merge(new_key: &[u8], existing_val: Option<&[u8]>,
-    mut operands: &mut MergeOperands) -> Vec<u8> {
+    operands: &mut MergeOperands) -> Vec<u8> {
     let mut result: Vec<u8> = Vec::with_capacity(operands.size_hint().0);
-    match existing_val {
-        Some(v) => result.push_all(v),
-        None => (),
-    }
+    existing_val.map(|v| { result.push_all(v) });
     for op in operands {
         result.push_all(op);
     }