minor #18079 Update upload_file.rst (hbgamra)

This PR was submitted for the 6.2 branch but it was merged into the 5.4 branch instead.

Discussion
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Update upload_file.rst

Fix return type string

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Commits
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ea0a512 Update upload_file.rst

Author: javiereguiluz

controller/upload_file.rst

@@ -253,7 +253,7 @@ logic to a separate service::
             $this->slugger = $slugger;
         }
 
-        public function upload(UploadedFile $file)
+        public function upload(UploadedFile $file): string
         {
             $originalFilename = pathinfo($file->getClientOriginalName(), PATHINFO_FILENAME);
             $safeFilename = $this->slugger->slug($originalFilename);
@@ -268,7 +268,7 @@ logic to a separate service::
             return $fileName;
         }
 
-        public function getTargetDirectory()
+        public function getTargetDirectory(): string
         {
             return $this->targetDirectory;
         }