Replaced space with underscore for proper doc generation (meta-pytorch#1256)

Summary:
Seems like spaces in argument names for function documentation remove everything but the last word which lead to confusion reading the docs as both equality and inequality constraints were simply listed as "constraints".

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## Motivation

Improve documentation

### Have you read the [Contributing Guidelines on pull requests](https://github.com/pytorch/botorch/blob/main/CONTRIBUTING.md#pull-requests)?

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Pull Request resolved: meta-pytorch#1256

Test Plan:
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## Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/pytorch/botorch, and link to your PR here.)

Reviewed By: saitcakmak

Differential Revision: D37082593

Pulled By: Balandat

fbshipit-source-id: 5b8d46c866dd572e7f0417b16385074655437f97

Author: jakobzeitler

botorch/optim/optimize.py

@@ -75,10 +75,10 @@ def optimize_acqf(
         raw_samples: The number of samples for initialization. This is required
             if `batch_initial_conditions` is not specified.
         options: Options for candidate generation.
-        inequality constraints: A list of tuples (indices, coefficients, rhs),
+        inequality_constraints: A list of tuples (indices, coefficients, rhs),
             with each tuple encoding an inequality constraint of the form
             `\sum_i (X[indices[i]] * coefficients[i]) >= rhs`
-        equality constraints: A list of tuples (indices, coefficients, rhs),
+        equality_constraints: A list of tuples (indices, coefficients, rhs),
             with each tuple encoding an inequality constraint of the form
             `\sum_i (X[indices[i]] * coefficients[i]) = rhs`
         nonlinear_inequality_constraints: A list of callables with that represent