Tweak MASM BNF grammar

Author: Colin Robertson

docs/assembler/masm/masm-bnf-grammar.md

@@ -81,7 +81,7 @@ The BNF grammar allows recursive definitions. For example, the grammar uses qual
 
 *`blockStatements`*\
   *`directiveList`*\
-  | **`.CONTINUE`** **`.IF`** *`cExpr`* ⟧\
+  | **`.CONTINUE`** ⟦ **`.IF`** *`cExpr`* ⟧\
   | **`.BREAK`** ⟦ **`.IF`** *`cExpr`* ⟧
 
 *`bool`*\