Fix 'instruction syntax document' link (WebAssembly#107)

Currently, the link to the instruction syntax document results in a 404.
This commit updates the link to point to a instructions.rst that exists
and hopefully is the correct one to link to.

Author: danbev

proposals/Exceptions.md

@@ -314,7 +314,7 @@ out of memory are implementation-defined.)
 ## Changes to the text format
 
 This section describes change in the [instruction syntax
-document](https://github.com/WebAssembly/spec/blob/master/document/core/instructions.rst).
+document](https://github.com/WebAssembly/spec/blob/master/document/core/text/instructions.rst).
 
 ### New instructions