Update nullish documentation to correct chaining order (#2457)

This follows on from 0c0e495 to keep the docs in sync.

Author: nickcampbell18

README.md

@@ -2361,7 +2361,7 @@ A convenience method that returns a "nullish" version of a schema. Nullish schem
 const nullishString = z.string().nullish(); // string | null | undefined
 
 // equivalent to
-z.string().optional().nullable();
+z.string().nullable().optional();
 ```
 
 ### `.array`

README_ZH.md

@@ -1623,7 +1623,7 @@ z.nullable(z.string());
 const nullishString = z.string().nullish(); // string | null | undefined
 
 // equivalent to
-z.string().optional().nullable();
+z.string().nullable().optional();
 ```
 
 ### `.array`

deno/lib/README.md

@@ -2358,7 +2358,7 @@ A convenience method that returns a "nullish" version of a schema. Nullish schem
 const nullishString = z.string().nullish(); // string | null | undefined
 
 // equivalent to
-z.string().optional().nullable();
+z.string().nullable().optional();
 ```
 
 ### `.array`