fix typos

src/packhead.h

@@ -49,8 +49,8 @@ struct PackHeader final {
     // enum { magic = UPX_MAGIC_LE32 };
     int version;
     int format; // executable format UPX_F_xxx
-    int method; // compresison method M_xxx
-    int level;  // compresison level 1..10
+    int method; // compression method M_xxx
+    int level;  // compression level 1..10
     unsigned u_len;
     unsigned c_len;
     unsigned u_adler;