Add missing bypass on BorrowExpression

In compacting grammar productions, the `BorrowExpression` production
needs to preserve the case that allows `&$EXPR`, but did not.  Let's
fix that.

Author: traviscross

src/expressions/operator-expr.md

@@ -58,7 +58,7 @@ r[expr.operator.borrow]
 r[expr.operator.borrow.syntax]
 ```grammar,expressions
 BorrowExpression ->
-    (`&`|`&&`) ( `mut` | (`raw` (`const` | `mut`))) Expression
+    (`&`|`&&`) ( `mut` | (`raw` (`const` | `mut`)))? Expression
 ```
 
 r[expr.operator.borrow.intro]