reword (binary ...) doc section

Signed-off-by: Roman Volosatovs <rvolosatovs@riseup.net>

Author: rvolosatovs

design/mvp/Explainer.md

@@ -1383,7 +1383,7 @@ f64canon ::= <core:f64> without the `nan:0x` case.
 
 The validation rules for `value` require the `val` to match the `valtype`.
 
-`(binary ...)` expression provides an alternative syntax allowing the binary contents
+The `(binary ...)` expression form provides an alternative syntax allowing the binary contents
 of the value definition to be written directly in the text format, analogous to data segments,
 avoiding the need to understand type information when encoding or decoding.