Fix typo (#61)

Author: dumindu

source/docs/e5.error_and_none_propagation.md

@@ -81,7 +81,7 @@ fn function_with_error() -> Result<u64, String> {
 
 ```rust
 // using `?`
-let x = function_with_error()?; // if Err, returns immidiately; if Ok(255), set x to 255
+let x = function_with_error()?; // if Err, returns immediately; if Ok(255), set x to 255
 
 // using `try!()`
 let x = try!(function_with_error());