message: Fix section headers in README
  Resolves
diff: @@ -58,9 +58,7 @@ will walk you through how to build from source on both macOS and Ubuntu.
  rustup override set $(<rustc-version)
  ```
  -#### Additional Linux Prerequisites
  -
  -##### Ubuntu
  +#### Ubuntu
  On Ubuntu, you need a few extra libraries to build Alacritty. Here's an `apt`
  command that should install all of them. If something is still found to be
  @@ -70,7 +68,7 @@ missing, please open an issue.
  apt-get install cmake libfreetype6-dev libfontconfig1-dev xclip
  ```
  -##### Arch Linux
  +#### Arch Linux
  On Arch Linux, you need a few extra libraries to build Alacritty. Here's a
  `pacman` command that should install all of them. If something is still found
  @@ -80,7 +78,7 @@ to be missing, please open an issue.
  pacman -S cmake freetype2 fontconfig xclip
  ```
  -##### Fedora
  +#### Fedora
  On Fedora, you need a few extra libraries to build Alacritty. Here's a `dnf`
  command that should install all of them. If something is still found to be
  @@ -90,7 +88,7 @@ missing, please open an issue.
  dnf install cmake freetype-devel fontconfig-devel xclip
  ```
  -##### openSUSE
  +#### openSUSE
  On openSUSE, you need a few extra libraries to build Alacritty. Here's
  a `zypper` command that should install all of them. If something is
  @@ -100,14 +98,14 @@ still found to be missing, please open an issue.
  zypper install cmake freetype-devel fontconfig-devel xclip
  ```
  -##### Slackware
  +#### Slackware
  Compiles out of the box for 14.2
  For copy & paste support (middle mouse button) you need to install xclip
  https://slackbuilds.org/repository/14.2/misc/xclip/?search=xclip
  -##### Void Linux
  +#### Void Linux
  On [Void Linux](https://voidlinux.eu), install following packages before compiling Alacritty:
  @@ -115,7 +113,7 @@ On [Void Linux](https://voidlinux.eu), install following packages before compili
  xbps-install cmake freetype-devel freetype expat-devel fontconfig xclip
  ```
  -##### FreeBSD
  +#### FreeBSD
  On FreeBSD, you need a few extra libraries to build Alacritty. Here's a `pkg`
  command that should install all of them. If something is still found to be
  @@ -125,9 +123,9 @@ missing, please open an issue.
  pkg install cmake freetype2 fontconfig xclip
  ```
  -##### Other
  +#### Other
  -If you build Alacritty on another Linux distribution, we would love some help
  +If you build Alacritty on another distribution, we would love some help
  filling in this section of the README.
  ### Building
  