Mistakes in VPR Placement Constraints docs

[TaoBi22]

Expected Behaviour

VPR should succeed when provided with the example placement constraints in the 'VPR Placement Constraints' documentation.

Current Behaviour

VPR throws the following: Error parsing start element tag (line: 9 col: 10). This is due to a missing closing bracket - adding this bracket still results in Start-end tags mismatch (line: 9 col: 13) being thrown as the self-closing tags are missing backslashes.

Also worth noting that the note under constraints file format has an unintended em-dash before the command line argument and colons around the word option, where it should be formatted as an option - I think this is just due to an erroneous space.

Possible Solution

Just requires the addition of an angle bracket and some backslashes for the XML - I've updated this locally already so I'll submit it in a PR.
I believe the incorrect section in the note should read :vpr:option:`--read_vpr_constraints` in the source, as in other doc files - I'll add this in the PR too.

Steps to Reproduce

1. Copy the placement constraints provided (here)[https://github.com/verilog-to-routing/vtr-verilog-to-routing/blob/7989197469e9650e45a00e035f67139edc296831/doc/src/vpr/placement_constraints.rst] into a constraint file
2. Run VPR, providing that constraint file with the --read_vpr_constraints

[vaughnbetz]

Fixed in the mentioned PR.