Fix grammar rules containing bounds

Author: fmease

src/items/type-aliases.md

@@ -4,7 +4,7 @@ r[items.type]
 r[items.type.syntax]
 ```grammar,items
 TypeAlias ->
-    `type` IDENTIFIER GenericParams? ( `:` TypeParamBounds )?
+    `type` IDENTIFIER GenericParams? ( `:` TypeParamBounds? )?
         WhereClause?
         ( `=` Type WhereClause?)? `;`
 ```

src/paths.md

@@ -68,7 +68,7 @@ GenericArgsBinding ->
     IDENTIFIER GenericArgs? `=` Type
 
 GenericArgsBounds ->
-    IDENTIFIER GenericArgs? `:` TypeParamBounds
+    IDENTIFIER GenericArgs? `:` TypeParamBounds?
 ```
 
 r[paths.expr.intro]

src/types/impl-trait.md

@@ -3,9 +3,9 @@ r[type.impl-trait]
 
 r[type.impl-trait.syntax]
 ```grammar,types
-ImplTraitType -> `impl` TypeParamBounds
+ImplTraitType -> `impl` TypeParamBounds?
 
-ImplTraitTypeOneBound -> `impl` TraitBound
+ImplTraitTypeOneBound -> `impl` TraitBound?
 ```
 
 r[type.impl-trait.intro]

src/types/trait-object.md

@@ -3,9 +3,9 @@ r[type.trait-object]
 
 r[type.trait-object.syntax]
 ```grammar,types
-TraitObjectType -> `dyn`? TypeParamBounds
+TraitObjectType -> TypeParamBounds | `dyn` TypeParamBounds?
 
-TraitObjectTypeOneBound -> `dyn`? TraitBound
+TraitObjectTypeOneBound -> TraitBound | `dyn` TraitBound?
 ```
 
 r[type.trait-object.intro]