Fix the composer installation instructions
Fix the composer installation instructions

Closes #1132
edge-soft/dompdf,dompdf/dompdf,nathanntg/dompdf,IvoPereira/dompdf,IvoPereira/dompdf,dompdf/dompdf,BrunoDeBarros/dompdf,IvoPereira/dompdf,nathanntg/dompdf,edge-soft/dompdf,BrunoDeBarros/dompdf