Fix spelling mistake on Cygwin

src/Composer/Installer/BinaryInstaller.php

@@ -197,7 +197,7 @@ class BinaryInstaller
 dir=\$(cd "\${0%[/\\\\]*}" > /dev/null; cd $binDir && pwd)
 
 if [ -d /proc/cygdrive ] && [[ \$(which php) == \$(readlink -n /proc/cygdrive)/* ]]; then
-   # We are in Cgywin using Windows php, so the path must be translated
+   # We are in Cygwin using Windows php, so the path must be translated
    dir=\$(cygpath -m "\$dir");
 fi