Fix small sphinx docs rendering issue

[kandersolar]

On the parallel algorithms docs page, there is an RST directive inside a literal block, which I assume must be unintentional:

Removing one of the trailing colons in the preceding line should allow the directive to be interpreted correctly instead of placed into a literal block, though I did not confirm this before opening the PR.

[inducer]

LGTM. Thanks a lot for the fix!