Replacing the special tick character (#168)

* Replacing the special tick character

The use of a special tick character in schema_isa.yaml cause
the riscof validateyaml command to fail in some environments.
This has been reported in the following issues.

https://github.com/riscv-software-src/riscof/issues/17
https://github.com/riscv-software-src/riscof/issues/107



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Signed-off-by: Neel Gala <neelgala@incoresemi.com>