fix: Fix syntax error in exceptions example. (#93)

* Update 04-exceptions.md

I spent so much time figuring out why my client app was giving errors since I was not experienced with custom exceptions on the client side, I was like why is it not working then I looked on Google and found out this Exception name doesn't need brackets, Please merge this, many new developers like me can be stuck and then stress why my code is not working.

* Update 04-exceptions.md

Added the fix in the docs folder same as added in the versioned docs folder for the Fix 
Refer #93 for the commits

docs/05-concepts/04-exceptions.md

@@ -41,10 +41,10 @@ In your app, catch the exception as you would catch any exception.
 try {
   client.example.doThingy();
 }
-on (MyException) catch(e) {
+on MyException catch(e) {
   print(e.message);
 }
 catch(e) {
   print('Something else went wrong.');
 }
-```
+```

versioned_docs/version-1.2.0/05-concepts/04-exceptions.md

@@ -41,10 +41,10 @@ In your app, catch the exception as you would catch any exception.
 try {
   client.example.doThingy();
 }
-on (MyException) catch(e) {
+on MyException catch(e) {
   print(e.message);
 }
 catch(e) {
   print('Something else went wrong.');
 }
-```
+```