fix(docs): update README examples to use File.read instead of StringIO for file uploads

Author: manu4543

README.md

@@ -529,7 +529,7 @@ When the library is unable to connect to the API, or if the API returns a non-su
 ```ruby
 begin
   file = image_kit.files.upload(
-    file: StringIO.new("https://www.example.com/public-url.jpg"),
+    file: File.read("/path/to/file.jpg"),
     file_name: "file-name.jpg"
   )
 rescue Imagekit::Errors::APIConnectionError => e
@@ -575,7 +575,7 @@ image_kit = Imagekit::Client.new(
 
 # Or, configure per-request:
 image_kit.files.upload(
-  file: StringIO.new("https://www.example.com/public-url.jpg"),
+  file: File.read("/path/to/file.jpg"),
   file_name: "file-name.jpg",
   request_options: {max_retries: 5}
 )
@@ -593,7 +593,7 @@ image_kit = Imagekit::Client.new(
 
 # Or, configure per-request:
 image_kit.files.upload(
-  file: StringIO.new("https://www.example.com/public-url.jpg"),
+  file: File.read("/path/to/file.jpg"),
   file_name: "file-name.jpg",
   request_options: {timeout: 5}
 )
@@ -628,7 +628,7 @@ Note: the `extra_` parameters of the same name overrides the documented paramete
 ```ruby
 response =
   image_kit.files.upload(
-    file: StringIO.new("https://www.example.com/public-url.jpg"),
+    file: File.read("/path/to/file.jpg"),
     file_name: "file-name.jpg",
     request_options: {
       extra_query: {my_query_parameter: value},
@@ -676,7 +676,7 @@ You can provide typesafe request parameters like so:
 
 ```ruby
 image_kit.files.upload(
-  file: StringIO.new("https://www.example.com/public-url.jpg"),
+  file: File.read("/path/to/file.jpg"),
   file_name: "file-name.jpg"
 )
 ```
@@ -686,13 +686,13 @@ Or, equivalently:
 ```ruby
 # Hashes work, but are not typesafe:
 image_kit.files.upload(
-  file: StringIO.new("https://www.example.com/public-url.jpg"),
+  file: File.read("/path/to/file.jpg"),
   file_name: "file-name.jpg"
 )
 
 # You can also splat a full Params class:
 params = Imagekit::FileUploadParams.new(
-  file: StringIO.new("https://www.example.com/public-url.jpg"),
+  file: File.read("/path/to/file.jpg"),
   file_name: "file-name.jpg"
 )
 image_kit.files.upload(**params)