fix CHANGES typo

Author: jgriffiths

CHANGES.md

@@ -5,7 +5,7 @@
 - Build: Note that the secp256k1-zkp library is now as git submodule rather
   than being directly checked in to the source tree. Run
   `git submodule sync --recursive` then `git submodule update --init --recursive`
-  from the srouce tree in order to clone the secp source and build the library.
+  from the source tree in order to clone the secp source and build the library.
   When you sync this change initially you may need to `rm -r src/secp256k1` then
   `git checkout src/secp256k1` to remove any old files and achieve a clean
   source tree.