doc: add missing backtick in buffer.markdown
doc: add missing backtick in buffer.markdown

Fixes: https://github.com/nodejs/node/issues/2880
PR-URL: https://github.com/nodejs/node/pull/2881
Reviewed-By: Jeremiah Senkpiel <f1782d15e31ad502ea2c097c21cc235811d0cb7c@rocketmail.com>

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