readme: Spelling error in bold in the first sentence

Nobody reads this, right?

README.md

@@ -11,7 +11,7 @@
 
 ## Goals
 
-Syncthing is a **continous file synchronization program**. It synchronizes
+Syncthing is a **continuous file synchronization program**. It synchronizes
 files between two or more computers. We strive to fulfill the goals below.
 The goals are listed in order of importance, the most important one being
 the first. This is the summary version of the goal list - for more