Fix typo in pages/docs/manual/latest/let-binding.mdx: %private -> %%private (#466)

Author: greyblake

pages/docs/manual/latest/let-binding.mdx

@@ -150,8 +150,8 @@ If you need _real_ mutation, e.g. passing a value around, have it modified by ma
 
 Private let bindings are introduced in the release [7.2](https://rescript-lang.org/blog/bucklescript-release-7-2).
 
-In the module system, everything is public by default, 
-the only way to hide some values is by providing a separate signature to 
+In the module system, everything is public by default,
+the only way to hide some values is by providing a separate signature to
 list public fields and their types:
 
 ```res
@@ -162,7 +162,7 @@ module A: {
   let b = 4
 }
 ```
-`%private` gives you an option to mark private fields directly
+`%%private` gives you an option to mark private fields directly
 
 ```res
 module A = {
@@ -171,15 +171,15 @@ module A = {
 }
 ```
 
-`%private` also applies to file level modules, so in some cases, 
+`%%private` also applies to file level modules, so in some cases,
 users do not need to provide a separate interface file just to hide some particular values.
 
-Note interface files are still recommended as a general best practice since they give you better 
-separate compilation units and also they're better for documentation. 
+Note interface files are still recommended as a general best practice since they give you better
+separate compilation units and also they're better for documentation.
 
-Still, `%private` is useful in the following scenarios:
+Still, `%%private` is useful in the following scenarios:
 
 - Code generators. Some code generators want to hide some values but it is sometimes very hard or time consuming for code generators to synthesize the types for public fields.
 
-- Quick prototyping. During prototyping, we still want to hide some values, but the interface file is not stable yet, `%private` provide you such convenience.
+- Quick prototyping. During prototyping, we still want to hide some values, but the interface file is not stable yet, `%%private` provide you such convenience.