Update text/0000-unsafe-extern-blocks.md

Co-authored-by: Waffle Maybe <[email protected]>

Author: Lokathor

text/0000-unsafe-extern-blocks.md

@@ -33,7 +33,7 @@ An extern function is declared with a `;` instead of a function body (similar to
 An extern static value is also declared with a `;` instead of an expression (similar to an associated const of a trait).
 In both cases, the actual function body or value is provided by whatever external source (which is probably not even written in Rust).
 
-When an `unsafe extern` block is used, all declarations within that `extern` block *should* have the `unsafe` or `safe` keywords as part of their signature.
+When an `unsafe extern` block is used, all declarations within that `extern` block *must* have the `unsafe` or `safe` keywords as part of their signature.
 If one of the two keywords is not explicitly provided, the declaration is assumed to be `unsafe`, and also a warning is generated.
 The `safe` keyword is a contextual keyword, it is currently only used within `extern` blocks.