Fix use of soft-hyphens in security-guide

The security-guide used two soft-hyphens in a CFLAGS string (soft-hyphen
encoded as <C2><AD> for the commit message):

CFLAGS="[...] -<C2><AD>D_FORTIFY_SOURCE=2 <C2><AD>O2 [...]"

The result looks something like this in a web browser:

CFLAGS="[...] -D_FORTIFY_SOURCE=2 O2 [...]"

If you copied the string from the browser into a text-editor, you'd get
the soft-hyphens back.

The correct string must look like this:

CFLAGS="[...] -D_FORTIFY_SOURCE=2 -O2 [...]"

Change-Id: I9fc1aa2a963d3c9afd93498e7ab91e5c4ac0c5aa