[R] Swap code solutions for read / write CSV (#192)

* Update reading_and_writing_data.Rmd

Swap code solutions for read_csv_arrow and write_csv_arrow as the solutions are provided in the wrong order at present

r/content/reading_and_writing_data.Rmd

@@ -239,7 +239,7 @@ test_that("read_ipc_stream chunk works as expected", {
 unlink("my_table.arrows")
 ```
 
-## Read CSV files 
+## Write CSV files  
 
 You want to write Arrow data to a CSV file.
 
@@ -254,7 +254,7 @@ test_that("write_csv_arrow chunk works as expected", {
 })
 ```
 
-## Write CSV files 
+## Read CSV files
 
 You want to read a CSV file.