Change order of prerequisites in Mac development guide

Xcode commandline tools need to be installed before ffmpeg is
as they’re used to compile it.

Running-Mastodon/Development-guide.md

@@ -53,18 +53,18 @@ These are self-contained instructions for setting up a development environment o
 
 ### Prerequisites
 
-- Get [Homebrew](https://brew.sh) and use it to install the other dependencies:
-
-	```
-	brew install imagemagick ffmpeg yarn postgresql redis rbenv nodejs
-	```
-
 - Get [Xcode](https://developer.apple.com/xcode/) Commandline tools
 
 	```
 	xcode-select install
 	```
 
+- Get [Homebrew](https://brew.sh) and use it to install the other dependencies:
+
+	```
+	brew install imagemagick ffmpeg yarn postgresql redis rbenv nodejs
+	```
+
 - Configure Rbenv:
 
 	```