fix docs title underline

Signed-off-by: Nathaniel <[email protected]>

Author: NathanielF

docs/source/index.rst

@@ -123,7 +123,7 @@ Instrumental Variable regression is an appropriate technique when you wish to es
 .. image:: _static/iv_reg2.png
 
 Inverse Propensity Score Weighting
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 Inverse Propensity Score Weighting is a technique used to correct selection effects in observational data by re-weighting observations to better reflect an as-if random allocation to treatment status. This helps recover unbiased causal effect estimates.
 
 .. image:: _static/propensity_weight.png