Update flow_control.md (#1591)

Fixed a typo and changed `essential` to `integral`.

Author: jonasclaes

src/flow_control.md

@@ -1,4 +1,4 @@
 # Flow of Control
 
-An essential part of any programming languages are ways to modify control flow:
+An integral part of any programming language are ways to modify control flow:
 `if`/`else`, `for`, and others. Let's talk about them in Rust.