[Doc] Minor: Fixes several formatting issues

Fixes several minor formatting issues in the [Continuous Compilation] [1] section.

[1]: http://spark.apache.org/docs/latest/building-spark.html#continuous-compilation

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Author: Cheng Lian <lian@databricks.com>

Closes #4316 from liancheng/fix-build-instruction-docs and squashes the following commits:

0a92e01 [Cheng Lian] Fixes several formatting issues