Documentation Fix

I noticed a very minor issue in the docs. I would have opened a PR instead of an issue with the fix, but I can't seem to find the source for the docs.

On this page: https://xilinx.github.io/XRT/master/html/build.html
Under section "Building the XRT Runtime", it says:

build.sh script builds for both Debug and Release profiles.

I think this is no longer accurate. At least on my end, I have to pass the -opt flag to get a Release build. Suggested change:

The build.sh script builds for the Debug profile by default. To get a Release build, pass the -opt flag.

The help for build.sh --help would also need to be updated, from:

[-dbg] Build debug library only (default)
[-opt] Build optimized library only (default)

to

[-dbg] Build debug library only (default)
[-opt] Build optimized library only

Like I said, very minor, but thought I'd let you know.

Locate the source for the published build.html page and the build.sh --help text, since the issue does not name their files. Compare both statements with the current default behavior, then verify that the documentation and help output consistently describe Debug as the default and -opt as the Release option.