doc: s/Satoshi/Bitcoin

Unit was wrong in the docs

Author: zkao

src/api.rs

@@ -84,7 +84,7 @@ pub trait ElectrumApi {
     /// Tries to fetch `count` block headers starting from `start_height`.
     fn block_headers(&self, start_height: usize, count: usize) -> Result<GetHeadersRes, Error>;
 
-    /// Estimates the fee required in **Satoshis per kilobyte** to confirm a transaction in `number` blocks.
+    /// Estimates the fee required in **Bitcoin per kilobyte** to confirm a transaction in `number` blocks.
     fn estimate_fee(&self, number: usize) -> Result<f64, Error>;
 
     /// Returns the minimum accepted fee by the server's node in **Bitcoin, not Satoshi**.