Book 1 Section 2.2 encoding issue

[htwong737]

Recommend to remind the people if they use the powershell which win10 has given during installing, the output image file then will be encoded as UTF-16 LE, which will cause mostly image viewers which claims to be able to open ppm file then cannot open it.
There are two resolutions.

1. Use cmd
2. Use the newest powershell
   These two terminal will let the file be encoded as UTF-8, and it can be opened and seen normally.

[hollasch]

Duplicate of #636