Fix typo in install.sh

install.sh

@@ -3,7 +3,7 @@
 # Set source and target directories
 powerline_fonts_dir=$( cd "$( dirname "$0" )" && pwd )
 
-# if an argument is given it is used to select wich fonts to install
+# if an argument is given it is used to select which fonts to install
 prefix="$1"
 
 find_command="find \"$powerline_fonts_dir\" \( -name '$prefix*.[o,t]tf' -or -name '$prefix*.pcf.gz' \) -type f -print0"