fixes for install-ploticus.sh

* fixed typos in comments
* cd into the script's directory before doing anything, to ensure that the
  binary is actually installed into the extension's folder (per the comment at
  the top)
* do not write into extension's PHP files, instead prompt the user to add
  an appropriate line to their LocalSettings

Change-Id: I1f7924ef846f6f5a8e4261b6c169d6f15553e0a0