Fix assignment priority

The assignment operator is lower priority than any other binary
operator.

Otherwise the following:

```tiger
a := b | c
```

Would be parsed as:

```tiger
(a := b) | c
```

Instead of the expected:

```tiger
a := (b | c)
```

grammar.js

@@ -7,12 +7,12 @@ function sepBy(sep, rule) {
 }
 
 const PREC = {
-  assign: 6,
-  multiplicative: 5,
-  additive: 4,
-  comparative: 3,
-  and: 2,
-  or: 1,
+  multiplicative: 6,
+  additive: 5,
+  comparative: 4,
+  and: 3,
+  or: 2,
+  assign: 1,
 };
 
 module.exports = grammar({