[MRG] docs: fix missing delimeter for left

[framunoz]

Types of changes

• Add backslash in mathematical environment.
  E.g.: from

  :math:`\left{\mathcal{N}(\mu,\Sigma)\right}_{i=1}^n`
  

  now is

  :math:`\left\{\mathcal{N}(\mu,\Sigma)\right\}_{i=1}^n`
  

• Delete spaces at the change of line.

Motivation and context / Related issue

In the documentation of ot.gaussian appears the following warning:

Missing or unrecognized delimiter for \left

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[qbarthelemy]

Online documentation doesn't seem to be updated after merging latest branches.

[framunoz]

Online documentation doesn't seem to be updated after merging latest branches.

Which version are you looking at? When you select the “Development” version 0.9.6dev the changes are shown.

[rflamary]

Yes teh stable version of the doc is updated only on release

[qbarthelemy]

Ok, thank you!