[MINOR] [Docs] typo in docs/configuration.md

[vectorijk]

<\code> end tag missing backslash in
docs/configuration.md{L308-L339}

ref #8795

[vectorijk]

cc @srowen @jaceklaskowski

[srowen]

Good catch. A quick search suggests that's all of them.

[SparkQA]

Test build #45928 has finished for PR 9715 at commit 4968e0c.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Merged to master/1.6