https://imagej.nih.gov/ij/docs/: Mixed-content warning and redirects

[paulmenzel]

Accessing Linux Installation, the browser warns about mixed-content as the URL for the favicon is not using HTTPS:

<link rel="shortcut icon" href="http://rsb.info.nih.gov/ij/favicon.ico">

PS: To save redirects, on all pages rsb.info.nih.gov could be updated to https://imagej.nih.gov/ij/favicon.ico.