Corrected unsafe character (#3166)

The double dash was apparently converted to a unicode long dash. Although this is in a comment, the mere existence of these characters triggers editors on Windows such as Notepad to insert extra characters at the beginning of file, which breaks the entire script

toolset/setup/linux/prerequisites.sh

@@ -32,7 +32,7 @@ sudo apt-get -qqy install -o Dpkg::Options::="--force-confdef" -o Dpkg::Options:
 
 sudo pip install colorama==0.3.1
 # Version 2.3 has a nice Counter() and other features
-# but it requires —-allow-external and -—allow-unverified
+# but it requires --allow-external and --allow-unverified
 sudo pip install progressbar==2.2
 sudo pip install requests