replace file system with filesystem to keep consistent

HACKING.md

@@ -36,9 +36,9 @@ $ make validate-examples
 ### Virtual schema http/FileSystem
 
 The `schema` validator uses a virtual [http/FileSystem](https://golang.org/pkg/net/http/#FileSystem) to load the JSON schema files for validating OCI images and/or manifests.
-The virtual file system is generated using the `esc` tool and compiled into consumers of the `schema` package so the JSON schema files don't have to be distributed along with and consumer binaries.
+The virtual filesystem is generated using the `esc` tool and compiled into consumers of the `schema` package so the JSON schema files don't have to be distributed along with and consumer binaries.
 
-Whenever changes are being done in any of the `schema/*.json` files, one must refresh the generated virtual file system.
+Whenever changes are being done in any of the `schema/*.json` files, one must refresh the generated virtual filesystem.
 Otherwise schema changes will not be visible inside `schema` consumers.
 
 Prerequisites:

schema/schema.go

@@ -45,7 +45,7 @@ var (
 	}
 )
 
-// FileSystem returns an in-memory file system including the schema files.
+// FileSystem returns an in-memory filesystem including the schema files.
 // The schema files are located at the root directory.
 func FileSystem() http.FileSystem {
 	return fs