Reword "must" to "can"

This is to match the original wording and to match the wording in
expr.match.guard.chains.or. Parentheses are not necessarily the only way
to work around this limitation. For example, you could sometimes use
curly braces, or lift parts out to a variable, or don't use chains.

Author: ehuss

src/expressions/if-expr.md

@@ -189,7 +189,7 @@ r[expr.if.chains.or]
 If any condition operand is a `let` pattern, then none of the condition operands can be a `||` [lazy boolean operator expression][expr.bool-logic] due to ambiguity and precedence with the `let` scrutinee.
 
 > [!EXAMPLE]
-> If a `||` expression is needed, then parentheses must be used. For example:
+> If a `||` expression is needed, then parentheses can be used. For example:
 >
 > ```rust
 > # let foo = Some(123);