fix some typos

MIGRATION_GUIDE.md

@@ -6,14 +6,14 @@ For slice to array reference changes of the form:
 fn function(value: &[u8]) { ... }
 ```
 
-tto:
+to:
 
 ```
 fn function(value: &[u8; 12]) { ... }
 ```
 
 In the case of the caller using array of the right size already,
-no changes need to be done. When the caller is using a subslice,
+no changes need to be done. But, when the caller is using a subslice,
 one can use the following construction, from:
 
 ```
@@ -31,6 +31,7 @@ fn caller() {
    let slice = &[....];
    function(<&[u8; 12]>::try_from(&slice[0..12]).unwrap());
 }
+```
 
 Note the .unwrap() is just one way to (not) handle the error, and the caller
 should integrate the failing `try_from` case with the error handling