Fix extra whitespace

src/ch12-01-accepting-command-line-arguments.md

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 ## Accepting Command Line Arguments
 
 Our first task is to have `greprs` accept its two command line arguments. There

src/ch12-02-reading-a-file.md

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 ## Reading a File
 
 Now that we have some variables containing the information that we need, let's

src/ch12-04-testing-the-librarys-functionality.md

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 ## Testing the Library's Functionality
 
 Writing tests for the core functionality of our code is now easier since we

src/ch12-05-working-with-environment-variables.md

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 ## Working with Environment Variables
 
 Let's add one more feature: case insensitive searching. In addition, this

src/ch12-06-writing-to-stderr-instead-of-stdout.md

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 ## Write to `stderr` Instead of `stdout`
 
 Right now, we're writing all of our output to the terminal with `println!`.